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Otsuka M.,US Space Telescope Science Institute | Meixner M.,US Space Telescope Science Institute | Meixner M.,Harvard - Smithsonian Center for Astrophysics | Riebel D.,Johns Hopkins University | And 3 more authors.
Astrophysical Journal | Year: 2011

We have estimated elemental abundances of the planetary nebula (PN) Hen2-436 in the Sagittarius (Sgr) spheroidal dwarf galaxy using ESO/VLT FORS2, Magellan/MMIRS, and Spitzer/IRS spectra. We have detected candidates of fluorine [F II]λ4790, krypton [Kr III]λ6826, and phosphorus [P II]λ7875 lines and successfully estimated the abundances of these elements ([F/H] = +1.23, [Kr/H] = +0.26, [P/H] = +0.26) for the first time. These elements are known to be synthesized by the neutron capture process in the He-rich intershell during the thermally pulsing asymptotic giant branch (AGB) phase. We present a relation between C, F, P, and Kr abundances among PNe and C-rich stars. The detections of these elements in Hen2-436 support the idea that F, P, Kr together with C are synthesized in the same layer and brought to the surface by the third dredge-up. We have detected N II and O II optical recombination lines (ORLs) and derived the N2+ and O2+ abundances. The discrepancy between the abundance derived from the oxygen ORL and that derived from the collisionally excited line is >1 dex. To investigate the status of the central star of the PN, nebula condition, and dust properties, we construct a theoretical spectral energy distribution (SED) model to match the observed SED with CLOUDY. By comparing the derived luminosity and temperature of the central star with theoretical evolutionary tracks, we conclude that the initial mass of the progenitor is likely to be ∼1.5-2.0 M ⊙ and the age is ∼3000 yr after the AGB phase. The observed elemental abundances of Hen2-436 can be explained by a theoretical nucleosynthesis model with a star of initial mass 2.25 M ⊙, Z = 0.008, and LMC compositions. We have estimated the dust mass to be 2.9×10-4 M ⊙ (amorphous carbon only) or 4.0×10-4 M ⊙ (amorphous carbon and polycyclic aromatic hydrocarbon). Based on the assumption that most of the observed dust is formed during the last two thermal pulses and the dust-to-gas mass ratio is 5.58 × 10-3, the dust mass-loss rate and the total mass-loss rate are <3.1×10-8 M ⊙ yr-1and <5.5×10-6 M ⊙ yr-1, respectively. Our estimated dust mass-loss rate is comparable to a Sgr dwarf galaxy AGB star with similar metallicity and luminosity. © 2011. The American Astronomical Society. All rights reserved.

Otsuka M.,Academia Sinica, Taiwan | Otsuka M.,US Space Telescope Science Institute | Kemper F.,Academia Sinica, Taiwan | Hyung S.,Chungbuk National University | And 5 more authors.
Astrophysical Journal | Year: 2013

We performed multiwavelength observations of the young planetary nebula (PN) M1-11 and obtained its elemental abundances, dust mass, and the evolutionary status of the central star. The AKARI/IRC, VLT/VISIR, and Spitzer/IRS spectra show features due to carbon-rich dust, such as the 3.3, 8.6, and 11.3 μm features due to polycyclic aromatic hydrocarbons (PAHs), a smooth continuum attributable to amorphous carbon, and the broad 11.5 and 30 μm features often ascribed to SiC and MgS, respectively. We also report the presence of an unidentified broad feature at 16-22 μm, similar to the feature found in Magellanic Cloud PNe with either C-rich or O-rich gas-phase compositions. We identify for the first time in M1-11 spectral lines at 8.5 (blended with PAH), 17.3, and 18.9 μm that we attribute to the C60 fullerene. This identification is strengthened by the fact that other Galactic PNe in which fullerenes are detected have similar central stars, similar gas-phase abundances, and a similar dust composition to M1-11. The weak radiation field due to the relatively cool central stars in these PNe may provide favorable conditions for fullerenes to survive in the circumstellar medium. Using the photoionization code CLOUDY, combined with a modified blackbody, we have fitted the 0.1-90 μm spectral energy distribution (SED) and determined the dust mass in the nebula to be 3.5 × 10-4 M ⊙. Our chemical abundance analysis and SED model suggest that M1-11 is perhaps a C-rich PN with C/O ratio in the gas phase of +0.19 dex, and that it evolved from a 1-1.5 M ⊙ star. © 2013. The American Astronomical Society. All rights reserved.

Time series of selected images of 17P/Holmes taken with an RC filter. All images have the standard orientation, that is, north is up and east to the left. At the bottom right in each image, solid vectors denote the orientation of the negative velocity of the comet, and dashed vectors show radial vectors outward from the solar direction. Credit: Yuna Grace Kwon et al., 2014. (Phys.org)—Periodic comet 17P/Holmes is usually a very faint object. However, during its historic outburst in October 2007, when the comet's coma expanded to a diameter greater than that of the sun, it became visible to the naked eye and was temporarily the largest object in our solar system. This event has drawn the attention of astronomers worldwide trying to investigate the comet's uncertain and changing nature. 17P/Holmes has been intensively monitored during its perihelion passage in 2014 and the results of these observations were detailed in a paper published Dec. 29, 2014 on the arXiv pre-print server. 17P/Holmes is a Jupiter-family comet discovered by Edwin Holmes in 1892. Although the comet's massive outburst event in 2007 was well-observed, very little is known about the long-term evolution in the activity of 17P/Holmes. The perihelion passage seven years later was expected to be a great occasion to study the comet in detail. Unfortunately, in March 2014, it passed its perihelion so far that there have been no published reports about the physical state of the comet during this passage. Now, a research team led by Yuna Grace Kwon of the Seoul National University in South Korea, reports its photometric monitoring observations of the comet over a period of nearly two years from May 2013 to March 2015, including the 2014 perihelion passage. To monitor the comet's activity, six ground-based telescopes were employed at five observatories: the Ishigakijima Astronomical Observatory (IAO), the Okayama Astrophysical Observatory (OAO), the Nishi-Harima Astronomical Observatory (NHAO), the Siding Spring Observatory (SSO), and the Bohyunsan Optical Astronomy Observatory (BOAO). Thanks to these observations, the researchers were able to detect the comet's circumnuclear dust coma and its feeble dust tail. They found out that the dust mass loss rate of 17P/Holmes' inner dust coma has been declining with increasing heliocentric distance. They also noticed that the dust production rate became equivalent to the level before the 2007 outburst. "The dust production rates around the 2014 perihelion passage were about five orders of magnitudes lower than the maximum value during the 2007 outburst, while they were comparable to that of the pre-outburst data," the researchers wrote in the paper. According to research team, this similarity suggests that the comet's activity was restored to its former state, although it has shown lingering activity for years when it was around its aphelion. They noted that the mass loss process yields valuable information for monitoring the activity of the 17P/Holmes. Besides determining dust production rates, the scientists also calculated the diurnal skin depth and growth timescale of the comet's dust mantle, inferring that it is now nearly five to seven centimeters thick and has been developing apace over the last seven years. "17P/Holmes spends more than half a revolution period beyond the heliocentric distance of three astronomical units. Taking into account the persistent activities of the comet around its aphelion it should have had favorable conditions in developing the dust mantle with continuously outward sublimating volatiles over quite large portions of its orbit," the paper reads. The researchers assume that the activity of 17P/Holmes is highly controlled by the formation and evolution of dust mantle. By monitoring the evolution of the fractional active area over the cometary surface, they concluded that the overall activity of 17P/Holmes has been significantly restrained during the 2014–2015 outbound orbits. The new results might also indicate that inactive or dormant comets are aided by the development of their dust mantles acquired over their evolutionary histories in the inner part of our solar system. Explore further: Mini-Comets within a comet lit up 17P/Holmes during megaoutburst More information: Monitoring Observations of the Jupiter-Family Comet 17P/Holmes during 2014 Perihelion Passage, arXiv:1512.08797 [astro-ph.EP] arxiv.org/abs/1512.08797 Abstract We performed a monitoring observation of a Jupiter-Family comet, 17P/Holmes, during its 2014 perihelion passage to investigate its secular change in activity. The comet has drawn the attention of astronomers since its historic outburst in 2007, and this occasion was its first perihelion passage since then. We analyzed the obtained data using aperture photometry package and derived the Afrho parameter, a proxy for the dust production rate. We found that Afrho showed asymmetric properties with respect to the perihelion passage: it increased moderately from 100 cm at the heliocentric distance r_h=2.6-3.1 AU to a maximal value of 185 cm at r_h = 2.2 AU (near the perihelion) during the inbound orbit, while dropping rapidly to 35 cm at r_h = 3.2 AU during the outbound orbit. We applied a model for characterizing dust production rates as a function of r_h and found that the fractional active area of the cometary nucleus had dropped from 20%-40% in 2008-2011 (around the aphelion) to 0.1%-0.3% in 2014-2015 (around the perihelion). This result suggests that a dust mantle would have developed rapidly in only one orbital revolution around the sun. Although a minor eruption was observed on UT 2015 January 26 at r_h = 3.0 AU, the areas excavated by the 2007 outburst would be covered with a layer of dust (

The planets were discovered by an international team of astronomers led by Bun'ei Sato of the Tokyo Institute of Technology. The researchers employed the Okayama Astrophysical Observatory (OAO) in Japan, the Xinglong Station in China and the Australian Astronomical Observatory (AAO) to observe HD 47366. The planets were detected by the radial velocity method, also known as Doppler spectroscopy, which uses gravity to detect exoworlds. The astronomers were searching for any signs of wobbling when observing HD 47366, as planets exert a gravitational tug when they orbit their parent stars, causing them to wobble back and forth. Three powerful spectrographs were needed to detect this wobbling: the HIgh Dispersion Echelle Spectrograph (HIDES) at OAO, the Coude Echelle Spectrograph (CES) at Xinglong and the University College London Echelle Spectrograph (UCLES) at AAO. Precise radial-velocity measurements using these spectrographs revealed the presence of two exoplanets orbiting HD 47366. By fitting a double Keplerian model to the obtained radial-velocity data, the researchers were able to determine the mass, semimajor axis and eccentricity of the newly discovered worlds. According to their computations, the inner and outer planet have minimum masses equal to 1.75 and 1.86 Jupiter masses, semimajor axes of 1.214 and 1.853 AU (astronomical units), and eccentricities of 0.089 and 0.278 respectively. With relatively small orbital separations, this planetary system has immediately become very intriguing for the scientists. "The planetary system is intriguing in the points that the best-fit Keplerian orbit is unstable, it is near but less likely in 2:1 mean-motion resonance, and could be stable if the orbits are nearly circular or in retrograde configuration," the researchers wrote in the paper. To further investigate the orbital stability of the system and constrain orbital parameters, the astronomers performed dynamical analysis for the system. This analysis revealed that the best-fit orbits in prograde configuration are unstable. However, the scientists found that they are stable in the following cases: The two planets are in the 2:1 mean-motion resonance; the eccentricity of the outer planet is less than about 0.15; mutual inclination of two the planets is larger than 160 degrees. The researchers also assume that the current orbital configuration could also be caused by a possible third planet in this system. However, no convincing evidence supporting this theory has yet been released. According to the research team, it is still unknown why multi-giant-planet systems with small orbital separation are mostly found around evolved, intermediate-mass stars. The scientists have offered one possible explanation for this phenomenon. "It may be a primordial property of planets around intermediate-mass stars that could be an outcome of planet formation or an acquired one as a result of orbital evolution caused by stellar evolution (stellar tide and mass loss) of central stars," the paper reads. To this date, precise radial-velocity surveys have found about 120 substellar companions around evolved stars. The discovery made by Sato and his team is another important finding increasing the population of multi-giant-planet systems found with relatively small orbital separations around evolved intermediate-mass stars. Planets around these type of stars, especially consisting of giant exoplanets, could be crucial for our understanding of formation and evolution of planetary systems. Explore further: Astronomers discover new planet in Pisces constellation More information: A Pair of Giant Planets around the Evolved Intermediate-Mass Star HD 47366: Multiple Circular Orbits or a Mutually Retrograde Configuration, arXiv:1601.04417 [astro-ph.EP] arxiv.org/pdf/1601.04417.pdf Abstract We report the detection of a double planetary system around the evolved intermediate-mass star HD 47366 from precise radial-velocity measurements at Okayama Astrophysical Observatory, Xinglong Station, and Australian Astronomical Observatory. The star is a K1 giant with a mass of 1.81+-0.13M_sun, a radius of 7.30+-0.33R_sun, and solar metallicity. The planetary system is composed of two giant planets with minimum mass of 1.75^{+0.20}_{-0.17}Mjup and 1.86^{+0.16}_{-0.15}Mjup, orbital period of 363.3^{+2.5}_{-2.4} d and 684.7^{+5.0}_{-4.9} d, and eccentricity of 0.089^{+0.079}_{-0.060} and 0.278^{+0.067}_{-0.094}, respectively, which are derived by a double Keplerian orbital fit to the radial-velocity data. The system adds to the population of multi-giant-planet systems with relatively small orbital separations, which are preferentially found around evolved intermediate-mass stars. Dynamical stability analysis for the system revealed, however, that the best-fit orbits are unstable in the case of a prograde configuration. The system could be stable if the planets were in 2:1 mean-motion resonance, but this is less likely considering the observed period ratio and eccentricity. A present possible scenario for the system is that both of the planets have nearly circular orbits, namely the eccentricity of the outer planet is less than ~0.15, which is just within 1.4sigma of the best-fit value, or the planets are in a mutually retrograde configuration with a mutual orbital inclination larger than 160 degree.

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