News Article | November 3, 2016
The detection of organic molecules with increasing complexity and potential biological relevance is opening the possibility to understand the formation of the building blocks of life in the interstellar medium. One of the families of molecules with astrobiological interest are the esters, whose simplest member, methyl formate, is rather abundant in star-forming regions. The next step in the chemical complexity of esters is ethyl formate, C2H5OCHO. Only two detections of this species have been reported so far, which strongly limits our understanding of how complex molecules are formed in the interstellar medium. We have searched for ethyl formate towards the W51 e2 hot molecular core, one of the most chemically rich sources in the Galaxy and one of the most promising regions to study prebiotic chemistry, especially after the recent discovery of the P−O bond, key in the formation of DNA. We have analyzed a spectral line survey towards the W51 e2 hot molecular core, which covers 44 GHz in the 1, 2 and 3 mm bands, carried out with the IRAM 30m telescope. We report the detection of the trans and gauche conformers of ethyl formate. A Local Thermodynamic Equilibrium analysis indicates that the excitation temperature is 78±10 K and that the two conformers have similar source-averaged column densities of (2.0±0.3)×1016 cm−2 and an abundance of ∼10−8. We compare the observed molecular abundances of ethyl formate with different competing chemical models based on grain surface and gas-phase chemistry. We propose that grain-surface chemistry may have a dominant role in the formation of ethyl formate (and other complex organic molecules) in hot molecular cores, rather than reactions in the gas phase. On the chemical ladder of esters. Detection and formation of ethyl formate in the W51 e2 hot molecular core Comments: Accepted in A&A; 11 pages, 6 figures, 7 Tables Subjects: Astrophysics of Galaxies (astro-ph.GA) Cite as: arXiv:1611.00719 [astro-ph.GA] (or arXiv:1611.00719v1 [astro-ph.GA] for this version) Submission history From: Victor Manuel Rivilla [v1] Wed, 2 Nov 2016 18:37:06 GMT (381kb) https://arxiv.org/abs/1611.00719
Guilloteau S.,University of Bordeaux 1 |
Guilloteau S.,French National Center for Scientific Research |
Dutrey A.,University of Bordeaux 1 |
Dutrey A.,French National Center for Scientific Research |
And 3 more authors.
Astronomy and Astrophysics | Year: 2011
Context. Proto-planetary disks are thought to provide the initial environment for planetary system formation. The dust and gas distribution and its evolution with time is one of the key elements in the process. Aims. We attempt to characterize the radial distribution of dust in disks around a sample of young stars from an observational point of view, and, when possible, in a model-independent way, by using parametric laws. Methods. We used the IRAM PdBI interferometer to provide very high angular resolution (down to 0.4′′ in some sources) observations of the continuum at 1.3 mm and 3 mm around a sample of T Tauri stars in the Taurus-Auriga region. The sample includes single and multiple systems, with a total of 23 individual disks. We used track-sharing observing mode to minimize the biases. We fitted these data with two kinds of models: a "truncated power law" model and a model presenting an exponential decay at the disk edge ("viscous" model). Results. Direct evidence for tidal truncation is found in the multiple systems. The temperature of the mm-emitting dust is constrained in a few systems. Unambiguous evidence for large grains is obtained by resolving out disks with very low values of the dust emissivity index β. In most disks that are sufficiently resolved at two different wavelengths, we find a radial dependence of β, which appears to increase from low values (as low as 0) at the center to about 1.7-2 at the disk edge. The same behavior could apply to all studied disks. It introduces further ambiguities in interpreting the brightness profile, because the regions with apparent β â‰̂ 0 can also be interpreted as being optically thick when their brightness temperature is high enough. Despite the added uncertainty on the dust absorption coefficient, the characteristic size of the disk appears to increase with a higher estimated star age. Conclusions. These results provide the first direct evidence of the radial dependence of the grain size in proto-planetary disks. Constraints of the surface density distributions and their evolution remain ambiguous because of a degeneracy with the β(r) law. © 2011 ESO.
News Article | March 1, 2017
Last year researchers "heard" black holes for the first time, when they detected the gravitational waves unleashed as two of them crashed together and merged. Now, they want to see a black hole, or at least its silhouette. Next month, astronomers will harness radio telescopes across the globe to create the equivalent of a single Earth-spanning dish—an instrument powerful enough, they hope, to image black holes backlit by the incandescent gas swirling around them. Their targets are the supermassive black hole at the heart of our Milky Way galaxy, known as Sagittarius A* (Sgr A*), and an even bigger one in the neighboring galaxy M87. Earlier observations using this Event Horizon Telescope (EHT) without its full roster of dishes yielded tantalizing results, but in images the two black holes remained featureless blobs. This year, for the first time, the EHT will add dishes in Chile and Antarctica, sharpening its resolution and raising expectations. Astronomers now hope to see how the black holes whip the hot gas around them into accretion disks and spawn matter-spewing jets. They also hope to chart the size and shape of the event horizon—the boundary of the black hole—to test whether Albert Einstein's theory of gravity, general relativity, still works under such extreme conditions. "It's a very bold and gutsy experiment," says theoretical astrophysicist Roger Blandford of Stanford University in Palo Alto, California, who is not involved in the project. Blandford believes the EHT may not only show how black holes work, but also deliver a more fundamental message. "It will validate this remarkable proposition: that black holes are common in the universe. Seeing is believing." The EHT takes aim just once a year, when good weather is likely, when both black holes are visible in the sky, and when it's possible to get time at all the observatories around the globe. This year, the team will observe for 5 nights during a 10-night window from 5 to 14 April. Then, an intensive data processing effort begins, and it may be a year before they know whether they've succeeded. "It's an exercise in delayed gratification. Delayed gratification squared," says EHT director Shep Doeleman at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. Imaging black holes is a formidable challenge, and not just because their intense gravity prevents even light from escaping. They are also surprisingly small. Sgr A* is calculated to contain the mass of 4 million suns, based on the nervy, high-speed orbits of stars in its gravitational grip. But its event horizon, the point of no return for anything approaching a black hole, is 24 million kilometers across, just 17 times wider than the sun. To see something so small from 26,000 light-years away requires a telescope dish of global dimensions. At optical wavelengths, Sgr A* is hidden by the shroud of dust and gas obscuring the galaxy's heart. Radio waves can pass through more easily, but ordinary radio dishes are still hampered by ionized gas clouds and low resolution. Best are telescopes sensitive to the shortest radio waves—millimeter waves—but the dishes, detectors, and data processing technology for this part of the spectrum were developed only in the past few decades. "There is only a tiny window where we can see the event horizon," says Heino Falcke, an astrophysicist at Radboud University in Nijmegen, the Netherlands, and chair of the EHT science council. "The Milky Way is like a milky glass." Early this decade, Doeleman and other EHT researchers began testing the idea with millimeter-sensitive dishes in Hawaii, California, and Arizona. Later, they extended the array to include the Large Millimeter Telescope in Mexico. Along the way, they got a good enough image of the black hole in M87 to see the base of its matter-spewing jets—data that are helping them understand how the jets are created. In 2015, they glimpsed the magnetic field around Sgr A*, which may help explain how black holes heat up the material around them. But to see the event horizon itself, the EHT had to grow even larger. Over the years, it has evolved from a loose, poorly funded group to a worldwide collaboration involving 30 institutions in 12 countries. Next month it will include farflung additions, including the IRAM dish in Spain, the South Pole Telescope, and the Atacama Large Millimeter/submillimeter Array (ALMA), a large international observatory comprising 66 dishes in northern Chile. With its huge dish area, ALMA is the big catch because it will boost the EHT's sensitivity by an order of magnitude. "That's the key for us," Doeleman says. Adding new instruments isn't simple. The technique for combining signals from distant dishes is known as very long baseline interferometry, and most millimeter-wave telescopes are not equipped to take part. EHT researchers had to visit each facility to tinker with its hardware and install new digital signal processors and data recorders. In the case of ALMA, that took some persuading. "We had to go into the bowels of ALMA and rewire it," Doeleman says. "It required political buy-in at all levels." The campaign next month will be a nervous time for the EHT team. All eight observatories need clear skies and no technical glitches to get the best possible observations. "The first time, things can go wrong," Falcke says. Data volumes will be so large that they have to be recorded on hard drives and shipped back to the Haystack Observatory in Westford, Massachusetts, and the Max Planck Institute for Radio Astronomy in Bonn, Germany, for processing. There, devices known as correlators, made from clusters of PCs but with the power of supercomputers, will spend months crunching through the data, combining the signals from separate dishes as if they came from a single dish as wide as Earth. Adding further delay, data from the South Pole Telescope won't arrive until September or October, when planes can retrieve the hard drives after the Antarctic winter. When the data finally all come together sometime next year, the team hopes to see a bright ring of light from photons orbiting close to the event horizon, with a dark disk in its center. The ring should be brighter on one side, where the rotation of the black hole gives photons a boost, although the images on this first attempt may not be as crisp as the team's simulations. "It'll probably be a crappy image, but scientifically it will be very interesting," Falcke says. Doeleman hopes to see structure in the matter swirling around the event horizon and watch, movielike, as gas falls into it and vanishes. Such observations might help explain why some black holes gorge on matter and shine brightly, whereas others—like Sgr A*—seem to be on a starvation diet. Falcke has a simpler wish. "The event horizon is the defining thing about a black hole," he says. "I hope to see it; to literally see it."
News Article | February 3, 2016
On the outskirts of the Rho Ophiuchi cloud complex, which is about 400 light-years from Earth, a Flying Saucer glows. Okay, the Flying Saucer isn’t a spaceship. It’s a young star officially known as 2MASS J16281370-2431391; its colloquially name is inspired by its appearance in visible pictures. But just because it isn’t an alien spacecraft doesn’t mean something interesting isn’t happening. In fact, an international team of scientists with the European Southern Observatory (ESO) recently glimpsed something they thought was physically impossible. Viewing the star’s protoplanetary disk—a collection of gas and dust associated with early planet formation—with the Atacama Large Millimeter/submillimeter Array (ALMA), the researchers observed and imaged the glow emanating from the disk’s carbon monoxide molecules. Surprisingly, the team observed a negative signal. “This disk is not observed against a black and empty night sky,” said Stephane Guilloteau, who is the lead author of a Letter to the Editor appearing in Astronomy & Astrophysics. “Instead it’s seen in silhouette in front of the glow of the Rho Ophiuchi Nebula. This diffuse glow is too extended to be detected by ALMA, but the disk absorbs it. The resulting negative signal means that parts of the disk are colder than the background. The Earth is quite literally in the shadow of the Flying Saucer!” The ALMA data was combined with other observations of the background glow from Spain’s IRAM 30-m telescope. With the two datasets, the researchers discovered the disk dust grain temperature was -266 C, colder than previous temperature models predicted. “Although dust is the main agent to control the protoplanetary disk temperature, our knowledge of dust temperatures essentially relies on modeling of disk images and (Spectral Energy Distribution),” the researchers write in Astronomy & Astrophysics. “Despite (or even because of) their sophistication, these models suffer from many uncertainties because of the large number of assumed properties: radial distribution, dust grain growth, dust settling, composition and porosity, disk flaring geometry, etc.” According to the ESO, most current models predict temperatures between -258 and -253 C. While the researchers haven’t pinpointed the exact reason behind the low temperature, they have a few ideas. One idea postulated is the temperature may depend on the disk’s grain sizes. The larger grains being cooler, and the smaller ones warmer. “It is too early to be sure,” said coauthor Emmanuel di Folco. Further observations are needed to understand the role temperature plays in planet formation.
News Article | February 3, 2016
This star is surrounded by a disc of gas and dust—such discs are called protoplanetary discs as they are the early stages in the creation of planetary systems. This particular disc is seen nearly edge-on, and its appearance in visible light pictures has led to its being nicknamed the Flying Saucer. The astronomers used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the glow coming from carbon monoxide molecules in the 2MASS J16281370-2431391 disc. They were able to create very sharp images and found something strange—in some cases they saw a negative signal! Normally a negative signal is physically impossible, but in this case there is an explanation, which leads to a surprising conclusion. Lead author Stephane Guilloteau takes up the story: "This disc is not observed against a black and empty night sky. Instead it's seen in silhouette in front of the glow of the Rho Ophiuchi Nebula. This diffuse glow is too extended to be detected by ALMA, but the disc absorbs it. The resulting negative signal means that parts of the disc are colder than the background. The Earth is quite literally in the shadow of the Flying Saucer!" The team combined the ALMA measurements of the disc with observations of the background glow made with the IRAM 30-metre telescope in Spain. They derived a disc dust grain temperature of only -266 degrees Celsius (only 7 degrees above absolute zero, or 7 Kelvin) at a distance of about 15 billion kilometres from the central star. This is the first direct measurement of the temperature of large grains (with sizes of about one millimetre) in such objects. This temperature is much lower than the -258 to -253 degrees Celsius (15 to 20 Kelvin) that most current models predict. To resolve the discrepancy, the large dust grains must have different properties than those currently assumed, to allow them to cool down to such low temperatures. "To work out the impact of this discovery on disc structure, we have to find what plausible dust properties can result in such low temperatures. We have a few ideas—for example the temperature may depend on grain size, with the bigger grains cooler than the smaller ones. But it is too early to be sure," adds co-author Emmanuel di Folco (Laboratoire d'Astrophysique de Bordeaux). If these low dust temperatures are found to be a normal feature of protoplanetary discs this may have many consequences for understanding how they form and evolve. For example, different dust properties will affect what happens when these particles collide, and thus their role in providing the seeds for planet formation. Whether the required change in dust properties is significant or not in this respect cannot yet be assessed. Low dust temperatures can also have a major impact for the smaller dusty discs that are known to exist. If these discs are composed of mostly larger, but cooler, grains than is currently supposed, this would mean that these compact discs can be arbitrarily massive, so could still form giant planets comparatively close to the central star. Further observations are needed, but it seems that the cooler dust found by ALMA may have significant consequences for the understanding of protoplanetary discs. This research was presented in a paper entitled "The shadow of the Flying Saucer: A very low temperature for large dust grains", by S. Guilloteau et al., published in Astronomy & Astrophysics Letters.
News Article | August 31, 2016
The galaxy cluster is called CL J1001+0220 (CL J1001 for short) and is located about 11.1 billion light years from Earth. The discovery of this object pushes back the formation time of galaxy clusters -the largest structures in the Universe held together by gravity - by about 700 million years. "This galaxy cluster isn't just remarkable for its distance, it's also going through an amazing growth spurt unlike any we've ever seen," said Tao Wang of the French Alternative Energies and Atomic Energy Commission (CEA) who led the study. The core of CL J1001 contains eleven massive galaxies - nine of which are experiencing an impressive baby boom of stars. Specifically, stars are forming in the cluster's core at a rate that is equivalent to over 3,000 Suns forming per year, a remarkably high value for a galaxy cluster, including those that are almost as distant, and therefore as young, as CL J1001. The diffuse X-ray emission detected by Chandra and ESA's XMM-Newton Observatory comes from a large amount of hot gas, one of the defining features of a true galaxy cluster. "It appears that we have captured this galaxy cluster at a critical stage just as it has shifted from a loose collection of galaxies into a young, but fully formed galaxy cluster," said co-author David Elbaz from CEA. Previously, only these loose collections of galaxies, known as protoclusters, had been seen at greater distances than CL J1001. The results suggest that elliptical galaxies in galaxy clusters like CL J1001 may form their stars during shorter and more violent outbursts than elliptical galaxies that are outside clusters. Also, this discovery suggests that much of the star formation in these galaxies happens after the galaxies fall onto the cluster, not before. In comparing their results to computer simulations of the formation of clusters performed by other scientists, the team of astronomers found that CL J1001 has an unexpectedly high amount of mass in stars compared to the cluster's total mass. This may show that the build-up of stars is more rapid in distant clusters than simulations imply, or it may show that clusters like CL J1001 are so rare that they are not found in today's largest cosmological simulations. "We think we're going to learn a lot about the formation of clusters and the galaxies they contain by studying this object," said co-author Alexis Finoguenov of the University of Helsinki in Finland, "and we're going to be searching hard for other examples." The result is based on data from a large group of observatories in space and on the ground including Chandra, NASA's Hubble Space Telescope and Spitzer Space Telescope, ESA's XMM-Newton and Herschel Space Observatory, the NSF's Karl G. Jansky Very Large Array, the Atacama Large Millimeter/submillimeter Array (ALMA), the Institut de Radioastronomie Millimetrique Northern Extended Millimeter Array (IRAM NOEMA), and ESO's Very Large Telescope. A paper describing these results will appear in The Astrophysical Journal on August 30th and is available online. NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations. Explore further: Chandra weighs most massive galaxy cluster in distant universe
News Article | February 4, 2016
A protoplanetary disk seen around a distant star, shaped like a typical flying saucer, is puzzling astronomers who recently recorded the temperature of the object. Particles within the structure are significantly cooler than current planetary models predict. The stellar body, known as 2MASS J16281370-2431391, sits inside the Rho Ophiuchi star-forming region. The star and its accompanying disk are located roughly 400 light-years away from our own planetary system. Images taken of the structure, sitting edge-on as seen from Earth, resemble a typical representation of an alien flying saucer. An international team of astronomers, including researchers from the Laboratoire d'Astrophysique de Bordeaux in France, recorded the temperature of the protoplanetary disk, finding it to be significantly cooler than expected - just 7 degrees Celsius above absolute zero. Protoplanetary disks are an early stage of planet formation. Most models of planetary formation suggest these particles - roughly 1.25 inch in length - should have a temperature of roughly 15 to 20 degrees above absolute zero. Examination of carbon monoxide in the structure, using the Atacama Large Millimeter/submillimeter Array (ALMA), recorded something unusual - a negative signal in certain parts of the disk. Such a result would normally be impossible. The findings were compared to observations made at the IRAM telescope in Spain. Astronomers realized the odd readings were the result of viewing the disk in front of the brighter nebula sitting behind the disk. "This diffuse glow is too extended to be detected by ALMA, but the disc absorbs it. The resulting negative signal means that parts of the disc are colder than the background. The Earth is quite literally in the shadow of the Flying Saucer!" Stephane Guilloteau of the Laboratoire d'Astrophysique de Bordeaux said. Astronomers are still unsure what to make of the findings. It is possible that larger particles of matter in these structures are cooler than smaller clumps of matter. How warm these pieces of matter are can affect the method by which they form new planets, asteroids, and comets. Analysis of the flying saucer structure and the unusually low temperature recorded for small particles in the structure was detailed in the journal Astronomy and Astrophysics.
News Article | September 7, 2016
ALMA Cycle-1 observations of the Orion Bar were carried out using 27 12 m antennae in band 7 at 345.796 GHz (CO J = 3–2) and 356.734 GHz (HCO+ J = 4–3). The observations consisted of a 27-pointing (the array points to 27 different positions to cover the field) mosaic centred at right ascension α(2000) = 5 h 35 min 20.6 s; declination δ(2000) = −05° 25′ 20′′. The total field-of-view is 58″ × 52″. Baseline configurations from about 12 m to about 444 m were used (C32-3 antennae configuration). Lines were observed with correlators providing a resolution of approximately 500 kHz (δv ≈ 0.4 km s−1) over a 937.5 MHz bandwidth. The total observation time on the ALMA 12 m array was around 2 h. ALMA executing blocks were first calibrated in the CASA software (version 4.2.0) and then exported to GILDAS. To recover the large-scale extended emission filtered out by the interferometer, we used fully sampled single-dish maps as ‘zero-’ and ‘short-spacings’. Maps were obtained with the IRAM 30 m telescope (Pico Veleta, Spain) using the EMIR330 receiver under excellent winter conditions (<1 mm of precipitable water vapour). On-the-fly scans of a 170″ × 170″ region were obtained both along and perpendicular to the Orion Bar. The beam full-width at half-maximum power (FWHM) at 350 GHz is about 7″. The GILDAS/MAPPING software was used to create the short-spacing visibilities31 not sampled by ALMA. These visibilities were merged with the interferometric observations. Each mosaic field was imaged and a dirty mosaic was built. The dirty image was deconvolved using the standard Högbom CLEAN algorithm and the resulting cubes were scaled from Jansky per beam to a brightness temperature scale using the synthesized beam size of about 1″. This resolution is a factor of approximately 9 higher than previous interferometric observations of the HCO+ J = 1–0 line towards the Orion Bar6. The achieved root mean squared noise is about 0.4 K per 0.4 km s−1 channel, with an absolute flux accuracy of about 10%. The resulting images are shown in Figs 1b and 2 and in Extended Data Fig. 2. Finally, the large-scale HCO+ J = 3–2 (267.558 GHz) on-the-fly map shown in Fig. 1a was taken with the multi-beam receiver HERA, also at the IRAM 30 m telescope. The spectral and angular resolutions are approximately 0.4 km s−1 and 9″ (FWHM) respectively. The final images were generated using the GILDAS/GREG software. To better understand the spatial distribution of the H v = 1–0 S(1) line emission at λ = 2.12 μm ( ) presented in ref. 1 and shown in Extended Data Fig. 2, we note two effects that determine the resulting emission morphology. First, there is a bright star in the line of sight towards the Orion Bar (Θ2AOri at α(2000) = 5 h 35 m 22.9 s; δ(2000) = −05° 24′ 57.8′′) that saturates the near-infrared detectors in a slit of width approximately 4″ parallel to the Orion Bar (roughly between δx = 19″ and 23″ in our rotated images). Hence, no data are shown in this range. Therefore, the layers with H vibrational emission are wider that suggested by Extended Data Fig. 2, and more emission peaks may coincide with HCO+ peaks in the blanked δx = 19″–23″ region. Older, near-infrared images with lower angular and spectral resolutions do show32 that the emission extends out to δx ≈ 20″. Second, dust extinction (due to foreground dust in Orion’s Veil and also due to dust in the Orion Bar itself) may affect the apparent morphology of the near-infrared images. Such effects are often neglected1, 32, 33 and are not included in Extended Data Fig. 2. The extinction towards the Orion Bar produced by the Veil is not greater than about 2 mag (ref. 34). Adopting a dust reddening appropriate to Orion11, 35, R = A /E(B − V) = 5.5, and the A /A (where B and V stand for the blue and visible photometric filters at 4,400 and 5,500 Å respectively, A and A are the extinctions in the visible and in the K filter at 2.2 μm, E(B − V)= A − A is the reddening factor, and R is a dimensionless parameter that characterizes the slope of the extinction curve) value in ref. 35, we estimate that the emission lines would only be approximately 30% brighter if foreground extinction corrections are taken into account. An additional magnitude of extinction due to dust in the atomic layer of the Bar itself results in a line intensity increase of about 50%. Therefore, minor morphological differences between the near-infrared and millimetre-wave images could reflect a small-scale or patchy extinction differences in the region1. To estimate the physical conditions of the HCO+-emitting gas near the dissociation front we run a grid of nonlocal, non-local thermodynamic equilibrium excitation and radiative transfer (Monte Carlo) models. This approach allows us to explore different column densities, gas temperatures and densities. Compared with most PDR models (using local escape probability approximations) our models take radiative pumping, line trapping and opacity broadening into account. This allows for the treatment of optically thick lines (see the appendix in ref. 36 for code details and benchmarking tests). Our models use the most recent inelastic collisional rates of HCO+ with H and with electrons, and of CO with both H and H. The electron density, n , is an important factor in the collisional excitation of molecular cations in a far-ultraviolet-illuminated gas. For HCO+, collisions with electrons start to contribute above n > 10 cm−3 (or n > 105 cm−3 if most of the electrons are provided by carbon atom ionization). In PDRs, collisions of molecules with H atoms can also contribute because the molecular gas fraction, f = 2n(H )/n = 2n(H )/[n(H)+2n(H )], is not 1 (a fully molecular gas). We adopted f = 0.8 and varied x between 0 and 10−4. The H ortho-to-para ratio was computed for each gas temperature T. Radiative excitation by the cosmic microwave background (T = 2.7 K) and by the far-infrared dust continuum in the Orion Bar37 (simulated by optically thin thermal emission at T = 55 K) were also included. Column densities of N(HCO+) = (5 ± 1) × 1013 cm−2 and N(CO) = (1.0 ± 0.5) × 1018 cm−2 were estimated using information from our IRAM 30-m telescope line-survey towards the dissociation front38. Several HCO+, H13CO+, HC18O+ and C18O rotational lines were included in the estimation (the quoted dispersions in the column densities reflect the uncertainty obtained from least square fits to rotational population diagrams). They are consistent with previous observations in the region5, 6. Radiative transfer models were run for N(HCO+) = 5 × 1013 cm−2, N(CO) = 1.0 × 1018 cm−2, and N = N(H) + 2N(H ) ≈ 2 × 1022 cm−2 (equivalent to A ≈ 7 mag for the dust properties in Orion). This results in x(HCO+) ≈ (2–3) ×10−9 and x(CO) ≈ (2.5–7.5) × 10−5 abundances. In addition, the HCO+/H13CO+ column density ratios derived from single-dish observations are similar to the 12C/13C = 67 isotopic ratio in Orion39. Thus, the H12CO+ lines are not very opaque (τ ≈ 2) otherwise the observed HCO+/H13CO+ line intensity ratios would be considerably smaller. A non-thermal (turbulent) velocity dispersion (σ ) of about 1 km s−1 reproduces the observed line widths. A similar value, 1.0–1.5 km s−1, is inferred directly from the observed line profiles ( = , with Δv = ≈ 3.0 ± 0.5 km s−1 and T = 300 K). Hence, opacity broadening plays a minor role. The dispersion σ is similar or lower than the local speed of sound at T = 100–300 K ( = 1.0–1.7 km s−1, where m is the mean mass per particle and k is the Boltzmann constant). This results in moderate Mach numbers M = σ /c ≤ 1. Extended Data Fig. 3 shows model predictions for the CO J = 3–2 line intensity peak, (upper left panel), and HCO+ J = 4–3 line integrated intensity, (where T is the line brightness temperature) (K km s−1), for different T and n values. For optically thick lines (τ 1), provides a good measure of the excitation temperature, with × (where T is the excitation temperature of the transition and E is the upper level energy). In addition, for low-critical-density (n ) transitions such as the low-J CO transitions, the lines are close to thermalization at densities above about 104 cm−3, thus (with n ≡ A /γ , where A is the Einstein coefficient for spontaneous emission and γ is the coefficient of the collisional de-excitation rate). In this case, is a good thermometer of the τ 1 emitting layers. The HCO+ J = 4–3 line, however, has much higher critical densities (n >5 × 106 cm−3 and n ≈ 103 e cm−3). For n < 2n /τ (sub-thermal excitation), the integrated line intensity is approximately linearly proportional to N(HCO+) = x(HCO+)n l (where l is the cloud length along the line of sight) even if the line is moderately thick. PDR models6, 7 and CO observations respectively show that x(HCO+) and T do not change substantially in the PDR layers around the emission peaks (cloud depths between A ≈ 1 and 2 mag). In a nearly edge-on PDR, the spatial length along the line of sight does not change greatly either. We compute that for the inferred T and N(HCO+) values in the region, the integrated line intensity is proportional to the density in the n = 104–106 cm−3 range (the correlation coefficient is r ≈ 0.98 for models with x = 0 and x = 10−4). Moreover, still increases with a density of up to several 106 cm−3 (r ≈ 0.94). This reasoning justifies the use of as a proxy for n in the region. The physical conditions that reproduce the mean CO J = 3–2 line peak and HCO+ J = 4–3 integrated line intensity towards the compressed structures at δx ≈ 15″ ( = 164 ± 10 K and = 69 ± 18 K km s−1) are T = 200–300 K and n = (1.0 ± 0.5) × 106 cm−3 (Extended Data Fig. 3). This implies high thermal pressures, P /k = n T ≈ (1.0–4.5) × 108 K cm−3 (where P is the pressure in the compressed gas component). The brightest HCO+ emission peaks (with , Fig. 2a) probably correspond to specific gas density enhancements. For the range of column densities and physical conditions at δx ≈ 15″, the gas temperature uncertainty is determined by the lack of higher-J CO lines, observed at high angular resolution, to better constrain T from excitation models. The range of estimated gas densities is dominated by the dispersion (about 25%) of the mean value. The above physical conditions suggest that the cloud edge contains substructures that are denser than the atomic layer3, 4 (n = (4–5) × 104 cm−3) and denser than the ambient molecular cloud5 (n = (0.5–1.0) × 105 cm−3). The equivalent length of the substructures is small, l = N /n ≈ (4–12) × 10−3 pc (where N is the total column density of hydrogen nuclei along the line of sight) (about 2′′–6″ at the distance to Orion, thus consistent with their apparent size in the ALMA image). The mass of a cylinder with n of a few 106 cm−3, 2′′–6″ length and width of 2″ is ≲0.005M (that is, a mass per unit length of (0.3–1.0)M pc−1). This is much lower than the virial and critical masses40 needed to make them gravitationally unstable (approximately 5M , from the inferred gas temperature, density and velocity dispersion). H clumps of similar small masses (several 0.001M ) have been intuited towards the boundary of more evolved and distant H ii regions41. Compression and fragmentation of ultraviolet-irradiated cloud edges must be a common phenomenon in the vicinity of young massive stars. Deeper inside the molecular cloud, smoothly decreases from about 170 K to about 130 K. Therefore, these observations do not suggest temperature spikes at scales of a few arcseconds. Deeper inside the molecular cloud (δx > 30″ in our rotated images), both N(H ) and N(HCO+) are expected to gradually increase5, 6, 7, 37. For the expected N(HCO+) ≈ 2 × 1014 cm−2 column density5, 6, excitation models show that the gas density in the ambient cloud is n ≈ (0.5–1.0) × 105 cm−3 (dashed curves in Extended Data Fig. 3), in agreement with previous estimations2, 5. Hence, the over-dense substructures have compression factors of approximately 5–30 with respect to the ambient molecular gas. The decrease of both and between the ionization and dissociation fronts is consistent with the expected sharp decrease of CO and HCO+ abundances in the atomic layer. The representative gas density in the atomic layer, n ≈ (4–5) × 104 cm−3, is constrained by the strength of the unattenuated far-ultraviolet flux at the Bar edge3, 5 (χ ≈ 4.4 × 104, determined by the spectral type of the Trapezium stars) and by the current position of the dissociation front at δx ≈ 15″ (refs 1 and 33). The exact gas density value, however, depends on the assumed far-ultraviolet-extinction grain properties (which probably vary as function of cloud depth). In the context of stationary PDR models, larger-than-standard-size grains (lower far-ultraviolet absorption cross-sections) are often invoked33, otherwise the separation between the dissociation and ionization fronts would be smaller than the observed around 15″. The lower densities in the atomic layer agree with the observed low H v = 1–0 S(1)/v = 2–1 S(1) ≈ 3 line intensity ratio attributed to fluorescent excitation32, 42. We note that optically thin CO emission implies . Hence, can no longer be used as a gas thermometer in the atomic layer where the CO abundance is low. The gas temperature close to the dissociation front is between T ≈ 500 K (from H i observations13) and T ≈ 300 K (from carbon radiorecombination43 and [C ii] 158 μm (ref. 11) line observations). To study the distribution of gas densities in the region, approximated by the HCO+ J = 4–3 emission, we analysed the probability distribution of the logarithmic emission, given by , where is the mean value in the observed field-of-view (37 K km s−1). This is a common approach used to interpret (column) density maps, both from observations and MHD simulations24, 44. The PDF is computed as the number of pixels (in the high signal-to-noise image) per intensity bin divided by the total number of pixels. We first analysed the complete field-of-view observed by ALMA and selected measurements above 5σ, where we define σ = rms(2δvΔv )1/2, with δv = 0.4 km s−1 and Δv = 3.0 km s−1. The resulting PDF is shown in Fig. 2d (magenta points). Second, we selected measurements only in the compressed layers region between δx = 7″ and 30″ (with respect to the rotated images in Fig. 2). The resulting PDF (black points) is very close to a log-normal distribution with , where z is the peak value and σ the standard deviation. We obtain z = 0.165 and σ = 0.31 from a fit (green curve). If is proportional to the gas density, these values imply that 99% of the observed positions in the compressed layers span a factor of about 6 in density. In MHD models, σ is a measure of how density varies in a turbulent cloud. Hence, it depends on the Mach number, the ratio of the thermal to magnetic pressure (β) and the forcing characteristics of the turbulence24. The relatively modest σ value inferred in the δx = 7″–30″ layer is consistent with the low Mach numbers in the PDR, and suggests an important role of magnetic pressure. We note that a similar analysis of the CO emission does not yield the same log-normal distribution. This is consistent with low-J CO lines being optically thick and tracing gas temperatures rather than gas density variations. This reinforces that the log-normal shape of the PDF in the compressed layer is a relevant observational result. To support the cloud compression and gas photoablation scenario, we investigated the different contributions to the gas pressure in the region. The thermal pressure in the H ii region near the ionization front1 is P , /k = 2n T ≈ 6 × 107 K cm−3, about six times higher than the turbulent ram pressure P = ρ in the ambient molecular cloud (Extended Data Table 1). As we find similar contributions from the thermal and non-thermal (turbulent) pressures in both the ambient cloud and the over-dense substructures (α = P /P ≈ P /P ≈ 1), it is reasonable to assume equipartition of thermal, turbulent and magnetic energies to quantify the magnetic pressure in the PDR (P = B2/8π). In particular, for β = P /P = 1 we estimate the magnetic field strengths B to be 200 μG and 800 μG in the ambient and in the high-density substructures, respectively. Such strong magnetic fields at small scales need to be confirmed observationally (both the strength and the orientation) but seem consistent with the high values (approximately 100 μG) measured in the low-density foreground material45 (the Orion Veil) confirming that B is particularly strong in the Orion complex. On much larger spatial scales, low-angular-resolution observations do suggest that B increases with density at H ii/cloud boundaries (B ∝ n 0.5−1) (ref. 46). A strong magnetic field would be associated with large magnetosonic speeds (v ) in the PDR. If an ultraviolet radiation-driven shockwave is responsible for the molecular gas compression, its velocity is predicted to slow down to v ≈ 3 km s−1 once it enters the molecular cloud21. In such a slow, magnetized shock (v ≪ v ), compression waves can travel ahead of the shock front47. Thus, a high magnetic field strength may be related to the undulations seen perpendicular to the Orion Bar (Fig. 2c). The inferred compression factor in the observed substructures (f = n /n = 5–30) is consistent with slow shock velocities16, v = c f 0.5 ≈ 1.5–4.0 km s–1, where c is the initial sound speed of the unperturbed molecular gas. The necessarily small v agrees with the relatively narrow molecular line-profiles (Δv ≤ 4 km−1) seen in PDRs14 (including observations of face-on sources in which the shock would propagate in the line of sight). Owing to the high thermal pressure in the compressed structures, we also find that a pressure gradient, with P ≥ P exists. This subtle effect is seen in simulations of an advancing shockwave around an H ii region22, 48. ALMA reveals fainter HCO+ and CO emission in the atomic layer (HCO+ globulettes and plume-like CO features at δx < 15″, Fig. 2). Previous low-angular-resolution observations and models had suggested the presence of dense spherical clumps with sizes of 5″–10″ deeper inside the molecular cloud6, 19 (at ≥ 15″–20″ from the ionization front3, 6, 32). The dense substructures resolved by ALMA are smaller (~2″ × 4″) and are detected at δx ≥ 7″ (even before the peak of the H vibrational emission). The molecular line profiles towards the plumes typically show two velocity emission components (Extended Data Fig. 4): one centred at v ≈ 8.5 km s−1 (where v refers to the emission velocity with respect to the local standard of rest), the velocity of the background molecular cloud in the back-side of M 42 (ref. 11; not directly associated with the Orion Bar), and other at v ≈ 11 km s−1, the velocity component of the molecular gas in the Orion Bar. In addition, despite the small size of the observed region, the crosscuts of the HCO+ J = 4–3 line velocity centroid and of the FWHM velocity dispersion show gradients perpendicular to the Orion Bar (Extended Data Fig. 4). Moving from the ionization front to the molecular gas, the line centroid shifts to higher velocities (gas compression effects may, in part, contribute to this redshifted velocity). The velocity dispersion, however, shows its maximum between the ionization and the dissociation fronts, the expected layers for photoablative neutral gas flows. Both the kinematic association with the Orion Bar velocities and the higher velocity dispersion between the two fronts are consistent with the presence of gas flowing from the high-pressure compressed molecular layers (P /k ≈ 2 × 108 K cm−3) to the atomic layers (P /k ≈ 5 × 107 K cm−3). Static equilibrium PDR models6 appropriate to the ambient gas component (n ≈ 5 × 104 cm−3) reproduce the separation between the ionization and dissociation fronts. However, they predict HCO+ abundances near the dissociation front that are too low (x(HCO+) of a few 5 × 10−11) to be consistent with the bright ridge of HCO+ emission detected by ALMA. These models also predict that the C+/CO transition should occur ahead of the H/H transition zone and deeper inside the molecular cloud (at δx ≈ 20″ from the ionization front3, 4). However, our detection of bright CO and HCO+ emission towards the layers of bright H vibrational emission1 implies that the C+/CO transition occurs closer to the cloud edge, and nearly coincides with the H/H transition (at least it cannot be resolved at the approximately 1″ resolution of our observations). This is probably another signature of dynamical effects. Indeed, the presence of molecular gas near the cloud edge49, and a reduced C+ abundance deeper inside the molecular cloud50, may explain model and observation discrepancies of other chemically related molecules. As an example, stationary PDR models applied to the fluorine chemistry51 overestimate the CF+ column density observed towards the Orion Bar52 by a factor of about 10. Given that HF readily forms as F atoms react with H molecules, CF+ must arise from layers where C+ and H overlap (CF+ forms through HF + C+ → CF+ + H reactions and is quickly destroyed by recombination with electrons)51, 53. Hence, the (lower-than-predicted) observed CF+ abundances probably reflect a dynamical PDR behaviour as well. Stationary PDR models of strongly irradiated dense gas (with n values of a few 106 cm−3) have been presented in the literature3, 6, 7. The above densities are similar to those inferred in the compressed substructures at the Orion Bar edge. Thus they can be used to gain insight into the chemistry that leads to the formation of HCO+ and CO in ultraviolet-irradiated dense gas. Owing to the higher densities and enhanced H collisional de-excitation heating, the gas attains high temperatures. This triggers a warm chemistry in which endothermic reactions and reactions with energy barriers become faster. As a result, higher HCO+ abundances are predicted close to the dissociation front (x(HCO+) of several 10−9). Reactions of C+ with H (either far-ultraviolet-pumped or thermally excited) initiate the carbon chemistry54. This reaction triggers the formation of CH+ (explaining the elevated CH+ abundances detected by Herschel55) and reduces the abundance of C+ ions and H molecules near the dissociation front; that is, the H/H and the C+/CO transition layers naturally get closer (in A )50. Fast exothermic reactions of CH+ with H subsequently produce CH + and CH +. Both hydrocarbon ions are ‘burnt’ in reactions with abundant oxygen atoms and contribute to the formation of HCO+ at the molecular cloud edge. This HCO+ formation route from CH+ can dominate over the formation of HCO+ from CO+ (after the O + H → OH + H reaction, followed by C+ + OH → CO+ + H, and finally CO+ + H → HCO+ + H)5, 6, 32. Both OH and CO+ have been detected in the Orion Bar56, 57, but high-angular-resolution maps do not exist. Recombination of HCO+ with electrons then drives CO production near the dissociation front6, 7. Extrapolating the above chemical scenario, the brightest HCO+ J = 4–3 emission peaks in the Orion Bar should be close to emission peaks. Extended Data Fig. 2a shows a remarkable spatial agreement between the H v = 1–0 S(1) emission peaks and several HCO+ emission peaks. Detailed H excitation models (including both far-ultraviolet-pumping and collisions) show that for the conditions prevailing in the Orion Bar, the intensity of the H v = 1–0 S(1) line is approximately proportional to the gas density42. Therefore, the HCO+ peaks that match the position of the H v = 1–0 S(1) line peaks probably correspond to gas density enhancements as well. This agrees with the higher H v = 1–0 S(1)/v = 2–1 S(1) ≈ 8 line intensity ratios observed at the dissociation front and consistent with efficient H collisional excitation32. The ALMA images thus confirm that in addition, or as a consequence of dynamical effects, reactions of H with abundant atoms and ions contribute to shift the molecular gas production towards the cloud edge. Even higher-angular-resolution observations of additional tracers will be needed to fully understand this, and to spatially resolve the chemical stratification expected in the over-dense substructures themselves. We note that if most of the carbon becomes CO at A ≈ 2 (N of several 1021 cm−2) in substructures with gas densities of a few 106 cm−3, this depth is equivalent to a spatial length of several 1015 cm, or an angular-scale of about 0.5″ at the distance to Orion. Deeper inside into the molecular cloud (δx > 30″), the CO+, CH+, CH + and CH + abundances sharply decrease. The far-ultraviolet flux greatly diminishes, and the gas and dust grain temperatures accordingly decrease. The HCO+ abundance also decreases until the CO + H + → HCO+ + H reaction starts to drive the HCO+ formation at low temperatures. Gas-phase atoms and molecules gradually deplete and dust grains become coated by ices as the far-ultraviolet photon flux is attenuated at even larger cloud depths (see Extended Data Fig. 1).
Andrews S.M.,Harvard - Smithsonian Center for Astrophysics |
Rosenfeld K.A.,Harvard - Smithsonian Center for Astrophysics |
Wilner D.J.,Harvard - Smithsonian Center for Astrophysics |
Astrophysical Journal Letters | Year: 2011
We present 870 μm observations of dust continuum emission from the LkCa 15 protoplanetary disk at high angular resolution (with a characteristic scale of 0″25 = 35AU), obtained with the IRAM Plateau de Bure interferometer and supplemented by slightly lower resolution observations from the Submillimeter Array. We fit these data with simple morphological models to characterize the spectacular ring-like emission structure of this disk. Our analysis indicates that a small amount of 870 μm dust emission (∼5mJy) originates inside a large (40-50AU radius) low optical depth cavity. This result can be interpreted either in the context of an abrupt decrease by a factor of ∼5 in the radial distribution of millimeter-sized dust grains or as indirect evidence for a gap in the disk, in agreement with previous inferences from the unresolved infrared spectrum and scattered light images. A preliminary model focused on the latter possibility suggests the presence of a low-mass (planetary) companion, having properties commensurate with those inferred from the recent discovery of LkCa 15b. © 2011. The American Astronomical Society. All rights reserved..
Gallerani S.,Normal School of Pisa |
Ferrara A.,Normal School of Pisa |
Neri R.,IRAM |
Maiolino R.,University of Cambridge
Monthly Notices of the Royal Astronomical Society | Year: 2014
We report the serendipitous detection of the CO(17-16) emission line towards the quasar sloan digital sky survey J114816.64+525150.3 (J1148) at redshift z ≃ 6.4 obtained with the Plateau de Bure Interferometer. The CO(17-16) line is possibly contaminated by OH+ emission, that may account for ∼35-60 per cent of the total flux observed. Photodissociation and X-ray-dominated regions (PDRs and XDRs) models show that PDRs alone cannot reproduce the high luminosity of the CO(17-16) line relative to low-J CO transitions and that XDRs are required. By adopting a composite PDR+XDR model, we derive molecular cloud and radiation field properties in the nuclear region of J1148. Our results show that highly excited CO lines represent a sensitive and possibly unique tool to infer the presence of X-ray faint or obscured supermassive black hole progenitors in high-z galaxies. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.