arXiv subjects

A. Hacar

Publications and source records attributed to A. Hacar.

50 records · Page 3Linked to original sources

arXiv2017

Fibers in the NGC1333 proto-cluster

Are the initial conditions for clustered star formation the same as for non-clustered star formation? To investigate the initial gas properties in young proto-clusters we carried out a comprehensive and high-sensitivity study of the internal structure, density, temperature, and kinematics of the dense gas content of the NGC1333 region in Perseus, one of the nearest and best studied embedded clusters. The analysis of the gas velocities in the Position-Position-Velocity space reveals an intricate underlying gas organization both in space and velocity. We identified a total of 14 velocity-coherent, (tran-)sonic structures within NGC1333, with similar physical and kinematic properties than those quiescent, star-forming (aka fertile) fibers previously identified in low-mass star-forming clouds. These fibers are arranged in a complex spatial network, build-up the observed total column density, and contain the dense cores and protostars in this cloud. Our results demonstrate that the presence of fibers is not restricted to low-mass clouds but can be extended to regions of increasing mass and complexity. We propose that the observational dichotomy between clustered and non-clustered star-forming regions might be naturally explained by the distinct spatial density of fertile fibers in these environments.

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arXiv2016

Orion revisited III. The Orion Belt population

This paper continues our study of the foreground population to the Orion molecular clouds. The goal is to characterize the foreground population north of NGC 1981 and to investigate the star formation history in the large Orion star-forming region. We focus on a region covering about 25 square degrees, centered on the $ε$ Orionis supergiant (HD 37128, B0\,Ia) and covering the Orion Belt asterism. We used a combination of optical (SDSS) and near-infrared (2MASS) data, informed by X-ray (\textit{XMM-Newton}) and mid-infrared (WISE) data, to construct a suite of color-color and color-magnitude diagrams for all available sources. We then applied a new statistical multiband technique to isolate a previously unknown stellar population in this region. We identify a rich and well-defined stellar population in the surveyed region that has about 2\,000 objects that are mostly M stars. We infer the age for this new population to be at least 5\, Myr and likely $\sim10$\,Myr and estimate a total of about 2\,500 members, assuming a normal IMF. This new population, which we call the Orion Belt population, is essentially extinction-free, disk-free, and its spatial distribution is roughly centered near $ε$ Ori, although substructure is clearly present. The Orion Belt population is likely the low-mass counterpart to the Ori OB Ib subgroup. Although our results do not rule out Blaauw's sequential star formation scenario for Orion, we argue that the recently proposed blue streams scenario provides a better framework on which one can explain the Orion star formation region as a whole. We speculate that the Orion Belt population could represent the evolved counterpart of an Orion nebula-like cluster.

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arXiv2016

Opacity broadening and interpretation of suprathermal CO linewidths: Macroscopic Turbulence and Tangled Molecular Clouds

(Abridged) Many of the observed CO line profiles exhibit broad linewidths that greatly exceed the thermal broadening expected within molecular clouds. These suprathermal CO linewidths are assumed to be originated from the presence of unresolved supersonic motions inside clouds. Typically overlooked in the literature, in this paper we aim to quantify the impact of the opacity broadening effects on the current interpretation of the CO suprathermal line profiles. Without any additional contributions to the gas velocity field, a large fraction of the apparently supersonic (${\cal M}\sim$2-3) linewidths measured in both $^{12}$CO and $^{13}$CO (J=1-0) lines can be explained by the saturation of their corresponding sonic-like, optically-thin C$^{18}$O counterparts assuming standard isotopic fractionation. Combined with the presence of multiple components detected in our C$^{18}$O spectra, these opacity effects seem to be also responsible of the highly supersonic linewidths (${\cal M}>$8-10) detected in the broadest $^{12}$CO and $^{13}$CO spectra in Taurus. Our results demonstrate that most of the suprathermal $^{12}$CO and $^{13}$CO linewidths could be primarily created by a combination of opacity broadening effects and multiple gas velocity components blended in these saturated emission lines. Once corrected by their corresponding optical depth, each of these gas components present transonic intrinsic linewidths consistently traced by the three CO isotopologues within a factor of 2. Highly correlated and velocity-coherent at large scales, the largest and highly supersonic velocity differences inside clouds are generated by the relative motions between individual gas components. This highly discretized structure of the molecular gas traced in CO suggest that the gas dynamics inside molecular clouds could be better described by the properties of a fully-resolved macroscopic turbulence.

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arXiv2016

APOGEE strings: a fossil record of the gas kinematic structure

We compare APOGEE radial velocities (RVs) of young stars in the Orion A cloud with CO line gas emission and find a correlation between the two at large-scales, in agreement with previous studies. However, at smaller scales we find evidence for the presence of substructure in the stellar velocity field. Using a Friends-of-Friends approach we identify 37 stellar groups with almost identical RVs. These groups are not randomly distributed but form elongated chains or strings of stars with five or more members with low velocity dispersion, across lengths of 1-1.5~pc. The similarity between the kinematic properties of the APOGEE strings and the internal velocity field of the chains of dense cores and fibers recently identified in the dense ISM is striking and suggests that for most of the Orion A cloud, young stars keep memory of the parental gas substructure where they originated.

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arXiv2015

The Musca cloud: A 6 pc-long velocity-coherent, sonic filament

Filaments play a central role in the molecular clouds' evolution, but their internal dynamical properties remain poorly characterized. To further explore the physical state of these structures, we have investigated the kinematic properties of the Musca cloud. We have sampled the main axis of this filamentary cloud in $^{13}$CO and C$^{18}$O (2--1) lines using APEX observations. The different line profiles in Musca shows that this cloud presents a continuous and quiescent velocity field along its $\sim$6.5 pc of length. With an internal gas kinematics dominated by thermal motions (i.e., $σ_{NT}/c_s\lesssim1$) and large-scale velocity gradients, these results reveal Musca as the longest velocity-coherent, sonic-like object identified so far in the ISM. The transonic properties of Musca present a clear departure from the predicted supersonic velocity dispersions expected in the Larson's velocity dispersion-size relationship, and constitute the first observational evidence of a filament fully decoupled from the turbulent regime over multi-parsec scales.

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arXiv2015

Filament Fragmentation in High-Mass Star Formation

Aims: We resolve the length-scales for filament formation and fragmentation (res. <=0.1pc), in particular the Jeans length and cylinder fragmentation scale. Methods: We observed the prototypical high-mass star-forming filament IRDC18223 with the Plateau de Bure Interferometer (PdBI) in the 3.2mm continuum and N2H+(1-0) line emission in a ten field mosaic at a spatial resolution of ~4'' (~14000AU). Results: The dust continuum emission resolves the filament into a chain of at least 12 relatively regularly spaced cores. The mean separation between cores is ~0.40(+-0.18)pc. While this is approximately consistent with the fragmentation of an infinite, isothermal, gravitationally bound gas cylinder, a high mass-to-length ratio of M/l~1000M_sun/pc requires additional turbulent and/or magnetic support against radial collapse of the filament. The N2H+(1-0) data reveal a velocity gradient perpendicular to the main filament. Although rotation of the filament cannot be excluded, the data are also consistent with the main filament being comprised of several velocity-coherent sub-filaments. Furthermore, this velocity gradient perpendicular to the filament resembles recent results toward Serpens south that are interpreted as signatures of filament formation within magnetized and turbulent sheet-like structures. Lower-density gas tracers ([CI] and C18O) reveal a similar red/blueshifted velocity structure on scales around 60'' east and west of the IRDC18223 filament. This may tentatively be interpreted as a signature of the large-scale cloud and the smaller-scale filament being kinematically coupled. We do not identify a velocity gradient along the axis of the filament. This may either be due to no significant gas flows along the filamentary axis, but it may partly also be caused by a low inclination angle of the filament with respect to the plane of the sky that could minimize such signature.

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arXiv2015

Velocity-resolved [CII] emission and [CII]/FIR Mapping along Orion with Herschel

We present the first 7.5'x11.5' velocity-resolved map of the [CII]158um line toward the Orion molecular cloud-1 (OMC-1) taken with the Herschel/HIFI instrument. In combination with far-infrared (FIR) photometric images and velocity-resolved maps of the H41alpha hydrogen recombination and CO J=2-1 lines, this data set provides an unprecedented view of the intricate small-scale kinematics of the ionized/PDR/molecular gas interfaces and of the radiative feedback from massive stars. The main contribution to the [CII] luminosity (~85%) is from the extended, FUV-illuminated face of the cloud G_0>500, n_H>5x10^3 cm^-3) and from dense PDRs (G_0~10^4, n_H~10^5 cm^-3) at the interface between OMC-1 and the HII region surrounding the Trapezium cluster. Around 15% of the [CII] emission arises from a different gas component without CO counterpart. The [CII] excitation, PDR gas turbulence, line opacity (from [13CII]) and role of the geometry of the illuminating stars with respect to the cloud are investigated. We construct maps of the [CII]/FIR and FIR/M_Gas ratios and show that [CII]/FIR decreases from the extended cloud component (10^-2-10^-3) to the more opaque star-forming cores (10^-3-10^-4). The lowest values are reminiscent of the "[CII] deficit" seen in local ultra-luminous IR galaxies hosting vigorous star formation. Spatial correlation analysis shows that the decreasing [CII]/FIR ratio correlates better with the column density of dust through the molecular cloud than with FIR/M_Gas. We conclude that the [CII] emitting column relative to the total dust column along each line of sight is responsible for the observed [CII]/FIR variations through the cloud.

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arXiv2014

Chains of dense cores in the Taurus L1495/B213 complex

(Abridged) We study the kinematics of the dense gas in the Taurus L1495/B213 filamentary region to investigate the mechanism of core formation. We use observations of N2H+(1-0) and C18O(2-1) carried out with the IRAM 30m telescope. We find that the dense cores in L1495/B213 are significantly clustered in linear chain-like groups about 0.5pc 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. 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. This scenario may also apply to other regions of star formation.

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arXiv2014

On the equilibrium of rotating filaments

The physical properties of the so-called Ostriker isothermal, non-rotating filament have been classically used as benchmark to interpret the stability of the filaments observed in nearby clouds. However, such static picture seems to contrast with the more dynamical state observed in different filaments. In order to explore the physical conditions of filaments under realistic conditions, in this work we theoretically investigate how the equilibrium structure of a filament changes in a rotating configuration. To do so, we solve the hydrostatic equilibrium equation assuming both uniform and differential rotations independently. We obtain a new set of equilibrium solutions for rotating and pressure truncated filaments. These new equilibrium solutions are found to present both radial and projected column density profiles shallower than their Ostriker-like counterparts. Moreover, and for rotational periods similar to those found in the observations, the centrifugal forces present in these filaments are also able to sustain large amounts of mass (larger than the mass attained by the Ostriker filament) without being necessary unstable. Our results indicate that further analysis on the physical state of star-forming filaments should take into account rotational effects as stabilizing agents against gravity

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arXiv2013

Non-isothermal filaments in equilibrium

The physical properties of the so-called Ostriker isothermal filament (Ostriker 1964) have been classically used as benchmark to interpret the stability of the filaments observed in nearby clouds. However, recent continuum studies have shown that the internal structure of the filaments depart from the isothermality, typically exhibiting radially increasing temperature gradients. The presence of internal temperature gradients within filaments suggests that the equilibrium configuration of these objects should be therefore revisited. The main goal of this work is to theoretically explore how the equilibrium structure of a filament changes in a non-isothermal configuration. We solve the hydrostatic equilibrium equation assuming temperature gradients similar to those derived from observations. We obtain a new set of equilibrium solutions for non-isothermal filaments with both linear and asymptotically constant temperature gradients. Our results show that, for sufficiently large internal temperature gradients, a non-isothermal filament could present significantly larger masses per unit length and shallower density profiles than the isothermal filament without collapsing by its own gravity. We conclude that filaments can reach an equilibrium configuration under non-isothermal conditions. Detailed studies of both the internal mass distribution and temperature gradients within filaments are then needed in order to judge the physical state of filaments.

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arXiv2013

HH 114 MMS: a new chemically active outflow

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 L1157 outflow. Methods. We have used the IRAM 30m 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.

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arXiv2013

Cores, filaments, and bundles: hierarchical core formation in the L1495/B213 Taurus region

(Abridged) Context. Core condensation is a critical step in the star-formation process, but 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. Results. From the N$_2$H$^+$ 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 C$^{18}$O 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 C$^{18}$O components appear as velocity-coherent structures, and we have identified them automatically using a dedicated algorithm (FIVe: Friends In Velocity). Using this algorithm, we have identified 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 a small number of fertile components with larger mass-per-unit-length being responsible for most cores in the cloud. At 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.

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arXiv2011

Dense core formation by fragmentation of velocity-coherent filaments in L1517

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+(1-0), and SO(JN=32-21) with the FCRAO 14m telescope, and in the 1.2mm dust continuum with the IRAM 30m 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.

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arXiv2010

A molecular survey of outflow gas: velocity-dependent shock chemistry and the peculiar composition of the EHV gas

(Abridged) We present a molecular survey of the outflows powered by L1448-mm and IRAS 04166+2706, two sources with prominent wing and extremely high velocity (EHV) components in their CO spectra. The molecular composition of the two outflows 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. We also find that the EHV regime is relatively rich in O-bearing species, as 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. We suggest that this difference in composition arises from a lower C/O ratio in the EHV gas. 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. The peculiar composition of the EHV gas may reflect its origin as a dense wind from the protostar or its surrounding disk.

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