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Johnstone D.,National Research Council Canada | Johnstone D.,University of Victoria | Rosolowsky E.,University of British Columbia | Tafalla M.,Observatorio Astronomico Nacional IGN | And 2 more authors.
Astrophysical Journal | Year: 2010

We investigate 35 prestellar cores and 36 protostellar cores in the Perseus molecular cloud. We find a very tight correlation between the physical parameters describing the N2H+ and NH3 gas. Both the velocity centroids and the line widths of N2H+ and NH3 correlate much better than either species correlates with CO, as expected if the nitrogen-bearing species are probing primarily the dense core gas where the CO has been depleted. We also find a tight correlation in the inferred abundance ratio between N2H+ and para-NH 3 across all cores, with N(p-NH3)/N(N2H +) = 22 ± 10. We find a mild correlation between NH 3 (and N2H+) column density and the (sub)millimeter dust continuum derived H2 column density for prestellar cores, N(p-NH3)/N(H2) ∼ 10-8, but do not find a fixed ratio for protostellar cores. The observations suggest that in the Perseus molecular cloud the formation and destruction mechanisms for the two nitrogen-bearing species are similar, regardless of the physical conditions in the dense core gas. While the equivalence of N2H + and NH3 as powerful tracers of dense gas is validated, the lack of correspondence between these species and the (sub)millimeter dust continuum observations for protostellar cores is disconcerting and presently unexplained. © 2010. The American Astronomical Society.

Balick B.,University of Washington | Gomez T.,University of Washington | Vinkovic D.,University of Split | Alcolea J.,Observatorio Astronomico Nacional IGN | And 3 more authors.
Astrophysical Journal | Year: 2012

We present four-color images of CRL 2688 obtained in 2009 using the Wide-Field Camera 3 on Hubble Space Telescope. The F606W image is compared with archival images in very similar filters to monitor the proper motions of nebular structure. We find that the bright N-S lobes have expanded uniformly by 2.5% and that the ensemble of rings has translated radially by 007 in 6.65yr. The rings were ejected every 100yr for 4 millennia until the lobes formed 250yr ago. Starlight scattered from the edges of the dark E-W dust lane is coincident with extant H2 images and leading tips of eight pairs of CO outflows. We interpret this as evidence that fingers lie within geometrically opposite cones of opening angles 30°like those in CRL618. By combining our results of the rings with 12COabsorption from the extended asymptotic giant branch (AGB) wind we ascertain that the rings were ejected at 18kms-1 with very little variation and that the distance to CRL 2688, , is 300-350pc. Our 2009 imaging program included filters that span 0.6-1.6 μm. We constructed a two-dimensional dust scattering model of stellar radiation through CRL 2688 that successfully reproduces the details of the nebular geometry, its integrated spectral energy distribution, and nearly all of its color variations. The model implies that the optical opacity of the lobes ≳ 1, the dust particle density in the rings decreases as radius-3, and that the mass and momentum of the AGBwinds and their rings have increased over time. © 2012. The American Astronomical Society. All rights reserved.

Balick B.,University of Washington | Huarte-Espinosa M.,University of Rochester | Frank A.,University of Texas at Austin | Gomez T.,University of Texas at Austin | And 4 more authors.
Astrophysical Journal | Year: 2013

Our ultimate goal is to probe the nature of the collimator of the outflows in the pre-planetary nebula CRL 618. CRL 618 is uniquely suited for this purpose owing to its multiple, bright, and carefully studied finger-shaped outflows east and west of its nucleus. We compare new Hubble Space Telescope images to images in the same filters observed as much as 11 yr ago to uncover large proper motions and surface brightness changes in its multiple finger-shaped outflows. The expansion age of the ensemble of fingers is close to 100 yr. We find strong brightness variations at the fingertips during the past decade. Deep IR images reveal a multiple ring-like structure of the surrounding medium into which the outflows propagate and interact. Tightly constrained three-dimensional hydrodynamic models link the properties of the fingers to their possible formation histories. We incorporate previously published complementary information to discern whether each of the fingers of CRL 618 are the results of steady, collimated outflows or a brief ejection event that launched a set of bullets about a century ago. Finally, we argue on various physical grounds that fingers of CRL 618 are likely to be the result of a spray of clumps ejected at the nucleus of CRL 618 since any mechanism that form a sustained set of unaligned jets is unprecedented. © 2013. The American Astronomical Society. All rights reserved.

Tafalla M.,Observatorio Astronomico Nacional IGN | Santiago-Garcia J.,Observatorio Astronomico Nacional IGN | Santiago-Garcia J.,Institute Radioastronomia Milimetrica IRAM | Hacar A.,Observatorio Astronomico Nacional IGN | Bachiller R.,Observatorio Astronomico Nacional IGN
Astronomy and Astrophysics | Year: 2010

Context. Bipolar outflows from Class 0 protostars often present two components in their CO spectra that have different kinematic behaviors: a smooth outflow wing and a discrete, extremely high-velocity (EHV) peak. Aims. To better understand the origin of these two outflow components, we have studied and compared their molecular composition. Methods. We carried out a molecular survey of the outflows powered by L1448-mm and IRAS 04166+2706, two sources with prominent wing and EHV components. For each source, we observed a number of molecular lines towards the brightest outflow position and used them to determine column densities for 12 different molecular species. Results. The molecular composition of the two outflows is very similar. It presents systematic changes with velocity that we analyze by dividing the outflow in three chemical regimes, two of them associated with the wing component and the other the EHV gas. The analysis of the two wing regimes shows that species like H2CO and CH3OH favor the low-velocity gas, while SiO and HCN are more abundant in the fastest gas. This fastest wing gas presents strong similarities with the composition of the "chemically active" L1157 outflow (whose abundances we re-evaluate in an appendix). We find that the EHV regime is relatively rich in O-bearing species compared to the wing regime. The EHV gas is not only detected in CO and SiO (already reported elsewhere), but also in SO, CH3OH, and H2CO (newly reported here), with a tentative detection in HCO+. At the same time, the EHV regime is relatively poor in C-bearing molecules like CS and HCN, for which we only obtain weak detections or upper limits despite deep integrations. We suggest that this difference in composition arises from a lower C/O ratio in the EHV gas. Conclusions. The different chemical compositions of the wing and EHV regimes suggest that these two outflow components have different physical origins. The wing component is better explained by shocked ambient gas, although none of the existing shock models explains all observed features. We hypothesize that the peculiar composition of the EHV gas reflects its origin as a dense wind from the protostar or its surrounding disk. © 2010 ESO.

Tafalla M.,Observatorio Astronomico Nacional IGN | Hacar A.,Observatorio Astronomico Nacional IGN | Hacar A.,University of Vienna
Astronomy and Astrophysics | Year: 2013

Context. A small group of bipolar protostellar outflows display strong emission from shock-tracer molecules such as SiO and CH3OH, and are generally referred to as "chemically active". The best-studied outflow from this group is the one in L 1157. Aims. We study the molecular emission from the bipolar outflow powered by the very young stellar object HH 114 MMS and compare its chemical composition with that of the L 1157 outflow. Methods. We have used the IRAM 30 m radio telescope to observe a number of transitions from CO, SiO, CH3OH, SO, CS, HCN, and HCO+ toward the HH 114 MMS outflow. The observations consist of maps and a two-position molecular survey. Results. The HH 114 MMS outflow presents strong emission from a number of shock-tracer molecules that dominate the appearance of the maps around the central source. The abundance of these molecules is comparable to the abundance in L 1157. Conclusions. The outflow from HH 114 MMS is a spectacular new case of a chemically active outflow. © ESO, 2013.

Baez-Rubio A.,CSIC - National Institute of Aerospace Technology | Martin-Pintado J.,CSIC - National Institute of Aerospace Technology | Thum C.,Institute Radio Astronomia Milimetrica IRAM | Planesas P.,Observatorio Astronomico Nacional IGN
Astronomy and Astrophysics | Year: 2013

Context. The best example of a massive star with an ionized outflow launched from its photoevaporating disk is MWC349A. The large amount of reported radio-continuum and radio-recombination line (RRL) observations toward this galactic UC-HII region offers a unique possibility to build a model of the ionized envelope of this source. Aims. To understand the physical conditions and kinematics of the ionized region of the circumstellar disk and also of the outflow of MWC349A. Methods. We compared the bulk of radio-continuum maps, RRL profiles, and the H30α centroid map published to date with the predictions of our non-LTE 3D radiative transfer model, MOdel for REcombination LInes (MORELI), which we describe here in detail. Results. Our non-LTE 3D radiative transfer model provides new evidence that the UC-HII region of MWC349A is composed of an ionized circumstellar disk rotating in Keplerian fashion around a star of 38 M™, and an ionized outflow expanding with a terminal velocity of 60 km s-1 and rotating in the same sense as the disk. The model shows that while maser amplification is the dominant process involved for Hnα RRL emission with quantum numbers n < 41, stimulated emission is relevant for the emission of RRLs with n > 41 up at least the H76α line. Conclusions. For the first time, we present a model of MWC349A which satisfactorily explains the vast amount of reported observational data for a very wide range of frequencies and angular resolutions. © 2013 ESO.

Hacar A.,Observatorio Astronomico Nacional IGN | Hacar A.,University of Vienna | Tafalla M.,Observatorio Astronomico Nacional IGN | Kauffmann J.,Jet Propulsion Laboratory | Kovacs A.,University of Minnesota
Astronomy and Astrophysics | Year: 2013

Context. Core condensation is a critical step in the star-formation process, but it is still poorly characterized observationally. Aims. We have studied the 10 pc-long L1495/B213 complex in Taurus to investigate how dense cores have condensed out of the lower density cloud material. Methods. We observed L1495/B213 in C18O(1-0), N2H+(1-0), and SO(JN = 32-21) with the 14 m FCRAO telescope, and complemented the data with dust continuum observations using APEX (870 μm) and IRAM 30 m (1200 μm). Results. From the N2H + emission, we identify 19 dense cores, some starless and some protostellar. They are not distributed uniformly, but tend to cluster with relative separations on the order of 0.25 pc. From the C18O emission, we identify multiple velocity components in the gas. We have characterized them by fitting Gaussians to the spectra and by studying the distribution of the fits in position-position-velocity space. In this space, the C18O components appear as velocity-coherent structures, and we identify them automatically using a dedicated algorithm (FIVE: Friends In VElocity). Using this algorithm, we identify 35 filamentary components with typical lengths of 0.5 pc, sonic internal velocity dispersions, and mass-per-unit length close to the stability threshold of isothermal cylinders at 10 K. Core formation seems to have occurred inside the filamentary components via fragmentation, with few fertile components with higher mass-per-unit length being responsible for most cores in the cloud. On large scales, the filamentary components appear grouped into families, which we refer to as bundles. Conclusions. Core formation in L1495/B213 has proceeded by hierarchical fragmentation. The cloud fragmented first into several pc-scale regions. Each of these regions later fragmented into velocity-coherent filaments of about 0.5 pc in length. Finally, a small number of these filaments fragmented quasi-statically and produced the individual dense cores we see today. © 2013 ESO.

Tafalla M.,Observatorio Astronomico Nacional IGN
EAS Publications Series | Year: 2016

Bipolar outflows result from the supersonic ejection of material by a protostar, and constitute one of the most characteristic signposts of stellar birth. They also provide ideal targets to test chemical models, and can serve as templates for more complex systems of galactic and extragalactic astronomy where supersonic interactions between gas components take place. © 2016 EAS, EDP Sciences.

Tafalla M.,Observatorio Astronomico Nacional IGN | Hacar A.,University of Vienna
Astronomy and Astrophysics | Year: 2015

Context. Cloud fragmentation into dense cores is a critical step in the process of star formation. A number of recent observations show that it is connected to the filamentary structure of the gas, but the processes responsible for core formation remain mysterious. Aims. We studied the kinematics and spatial distribution of the dense gas in the L1495/B213 filamentary region of the Taurus molecular cloud with the goal of understanding the mechanism of core formation. Methods. We mapped the densest regions of L1495/B213 in N2H+(1-0) and C18O(2-1) with the IRAM 30 m telescope, and complemented these data with archival dust-continuum observations from the Herschel Space Observatory. Results. The dense cores in L1495/B213 are significantly clustered in linear chain-like groups about 0.5 pc long. The internal motions in these chains are mostly subsonic and the velocity is continuous, indicating that turbulence dissipation in the cloud has occurred at the scale of the chains and not at the smaller scale of the individual cores. The chains also present an approximately constant abundance of N2H+ and radial intensity profiles that can be modeled with a density law that follows a softened power law. A simple analysis of the spacing between the cores using an isothermal cylinder model indicates that the cores have likely formed by gravitational fragmentation of velocity-coherent filaments. Conclusions. Combining our analysis of the cores with our previous study of the large-scale C18O emission from the cloud, we propose a two-step scenario of core formation in L1495/B213. In this scenario, named "fray and fragment", L1495/B213 originated from the supersonic collision of two flows. The collision produced a network of intertwined subsonic filaments or fibers (fray step). Some of these fibers accumulated enough mass to become gravitationally unstable and fragment into chains of closely-spaced cores. © ESO 2015.

Hacar A.,Observatorio Astronomico Nacional IGN | Tafalla M.,Observatorio Astronomico Nacional IGN
Astronomy and Astrophysics | Year: 2011

Context. Low-mass star-forming cores differ from their surrounding molecular cloud in turbulence, shape, and density structure. Aims. We aim to understand how dense cores form out of the less dense cloud material by studying the connection between these two regimes. Methods. We observed the L1517 dark cloud in C18O(1-0), N2H+(J = 1-0), and SO(JN = 32-21) with the FCRAO 14 m telescope, and in the 1.2 mm dust continuum with the IRAM 30 m telescope. Results. Most of the gas in the cloud lies in four filaments that have typical lengths of 0.5 pc. Five starless cores are embedded in these filaments and have chemical compositions indicative of different evolutionary stages. The filaments have radial profiles of C18O(1-0) emission with a central flattened region and a power-law tail, and can be fitted approximately as isothermal, pressure-supported cylinders. The filaments, in addition, are extremely quiescent. They have subsonic internal motions and are coherent in velocity over their whole length. The large-scale motions in the filaments can be used to predict the velocity inside the cores, indicating that core formation has not decoupled the dense gas kinematically from its parental material. In two filaments, these large-scale motions consist of oscillations in the velocity centroid, and a simple kinematic model suggests that they may be related to core-forming flows. Conclusions. Core formation in L1517 seems to have occurred in two steps. First, the subsonic, velocity-coherent filaments have condensed out of the more turbulent ambient cloud. Then, the cores fragmented quasi-statically and inherited the kinematics of the filaments. Turbulence dissipation has therefore occurred mostly on scales on the order of 0.5 pc or larger, and seems to have played a small role in the formation of the individual cores. © 2011 ESO.

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