Context. Mid- and far-infrared observations of the environment around embedded protostars reveal a plethora of high excitation molecular and atomic emission lines. Different mechanisms for the... Show moreContext. Mid- and far-infrared observations of the environment around embedded protostars reveal a plethora of high excitation molecular and atomic emission lines. Different mechanisms for the origin of these lines have been proposed, including shocks induced by protostellar jets and radiation or heating by the embedded protostar of its immediate surroundings. Aims: By studying of the most important molecular and atomic coolants, we aim at constraining the physical conditions around the embedded protostars SMM3 and SMM4 in the Serpens molecular cloud core and measuring the CO/H$_{2}$ ratio in warm gas. Methods: Spectro-imaging observations from the Spitzer Infrared Spectrograph (IRS) and the Herschel Photodetector Array Camera and Spectrometer (PACS) provide an almost complete wavelength coverage between 5 and 200 {$μ$}m. Within this range, emission from all major molecular (H$_{2}$, CO, H$_{2}$O and OH) and many atomic ([OI], [CII], [FeII], [SiII] and [SI]) coolants of excited gas are detected. Emission line maps reveal the morphology of the observed emission and indicate associations between the different species. The excitation conditions for molecular species are assessed through rotational diagrams. Emission lines from major coolants are compared to the results of steady-state C- and J-type shock models. Results: Line emission tends to peak at distances of ~{}10-20{Prime} from the protostellar sources with all but [CII] peaking at the positions of outflow shocks seen in near-IR and sub-millimeter interferometric observations. The [CII] emission pattern suggests that it is most likely excited from energetic UV radiation originating from the nearby flat-spectrum source SMM6. Excitation analysis indicates that H$_{2}$ and CO originate in gas at two distinct rotational temperatures of ~{}300 K and 1000 K, while the excitation temperature for H$_{2}$O and OH is ~{}100-200 K. The morphological and physical association between CO and H$_{2}$ suggests a common excitation mechanism, which allows direct comparisons between the two molecules. The CO/H$_{2}$ abundance ratio varies from ~{}10$^{-5}$ in the warmer gas up to ~{}10$^{-4}$ in the hotter regions. Shock models indicate that C-shocks can account for the observed line intensities if a beam filling factor and a temperature stratification in the shock front are considered. C-type shocks can best explain the emission from H$_{2}$O. The existence of J-shocks is suggested by the strong atomic/ionic (except for [CII]) emission and a number of line ratio diagnostics. Dissociative shocks can account for the CO and H$_{2}$ emission in a single excitation temperature structure. Conclusions: The bulk of cooling from molecular and atomic lines is associated with gas excited in outflow shocks. The strong association between H$_{2}$ and CO constrain their abundance ratio in warm gas. Both C- and J-type shocks can account for the observed molecular emission; however, J-shocks are strongly suggested by the atomic emission and provide simpler and more homogeneous solutions for CO and H$_{2}$. The variations in the CO/H$_{2}$ abundance ratio for gas at different temperatures can be interpreted by their reformation rates in dissociative J-type shocks, or the influence of both C and J shocks. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendices A-C are available in electronic form at http://www.aanda.orgShow less
Observations of the high redshift Universe through narrow-band filters have proven very successful in the last decade. The 4-m VISTA telescope, equipped with the wide-field camera VIRCAM, offers a... Show moreObservations of the high redshift Universe through narrow-band filters have proven very successful in the last decade. The 4-m VISTA telescope, equipped with the wide-field camera VIRCAM, offers a major step forward in wide-field near-infrared imaging, and in order to utilise VISTA's large field-of-view and sensitivity, the Dark Cosmology Centre provided a set of 16 narrow-band filters for VIRCAM. These NB118 filters are centered at a wavelength near 1.19 {$μ$}m in a region with few airglow emission lines. The filters allow the detection of H{$α$} emitters at z = 0.8, H{$β$} and [O iii] emitters at z {ap} 1.4, [O ii] emitters at z = 2.2, and Ly{$α$} emitters at z = 8.8. Based on guaranteed time observations of the COSMOS field we here present a detailed description and characterization of the filters and their performance. In particular we provide sky-brightness levels and depths for each of the 16 detector/filter sets and find that some of the filters show signs of some red-leak. We identify a sample of 2 { imes} 10$^{3}$ candidate emission-line objects in the data. Cross-correlating this sample with a large set of galaxies with known spectroscopic redshifts we determine the ''in situ'' passbands of the filters and find that they are shifted by about 3.5 - 4 nm (corresponding to 30% of the filter width) to the red compared to the expectation based on the laboratory measurements. Finally, we present an algorithm to mask out persistence in VIRCAM data. Scientific results extracted from the data will be presented separately. Based on observations collected at the European Southern Observatory, Chile, as part of programme 284.A-5026 (VISTA NB118 GTO, PI Fynbo) and 179.A-2005 (UltraVISTA, PIs Dunlop, Franx, Fynbo, {amp} Le F{è}vre). Show less
Aims: The aim of this work is to gain a deeper insight into how much different aggregate types are affected by erosion. Especially, it is important to study the influence of the velocity of the... Show moreAims: The aim of this work is to gain a deeper insight into how much different aggregate types are affected by erosion. Especially, it is important to study the influence of the velocity of the impacting projectiles. We also want to provide models for dust growth in protoplanetary disks with simple recipes to account for erosion effects. Methods: To study the erosion of dust aggregates we employed a molecular dynamics approach that features a detailed micro-physical model of the interaction of spherical grains. For the first time, the model has been extended by introducing a new visco-elastic damping force, which requires a proper calibration. Afterwards, different sample generation methods were used to cover a wide range of aggregate types. Results: The visco-elastic damping force introduced in this work turns out to be crucial to reproduce results obtained from laboratory experiments. After proper calibration, we find that erosion occurs for impact velocities of 5 ms$^{-1}$ and above. Though fractal aggregates as formed during the first growth phase are most susceptible to erosion, we observe erosion of aggregates with rather compact surfaces as well. Conclusions: We find that bombarding a larger target aggregate with small projectiles results in erosion for impact velocities as low as a few ms$^{-1}$. More compact aggregates suffer less from erosion. With increasing projectile size the transition from accretion to erosion is shifted to higher velocities. This allows larger bodies to grow through high velocity collisions with smaller aggregates. Show less
Herranz, D.; González-Nuevo, J.; Clements, D.; De Zotti, G.; Lopez-Caniego, M.; Lapi, A.; ... ; Maddox, S. 2013
We present the results of a cross-correlation of the Planck Early Release Compact Source catalogue (ERCSC) with the catalogue of Herschel-ATLAS sources detected in the phase 1 fields, covering 134... Show moreWe present the results of a cross-correlation of the Planck Early Release Compact Source catalogue (ERCSC) with the catalogue of Herschel-ATLAS sources detected in the phase 1 fields, covering 134.55{deg}$^{2}$. There are 28 ERCSC sources detected by Planck at 857 GHz in this area. As many as 16 of them are probably high Galactic latitude cirrus; 10 additional sources can be clearly identified as bright, low-z galaxies; one further source is resolved by Herschel as two relatively bright sources; and the last is resolved into an unusual condensation of low-flux, probably high-redshift point sources, around a strongly lensed Herschel-ATLAS source at z = 3.26. Our results demonstrate that the higher sensitivity and higher angular resolution H-ATLAS maps provide essential information for the interpretation of candidate sources extracted from Planck sub-mm maps. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA Show less
Yatawatta, S.; Bruyn, A.; Brentjens, M.; Labropoulos, P.; Pandey, V.; Kazemi, S.; ... ; Zarka, P. 2013
Aims: The aim of the LOFAR epoch of reionization (EoR) project is to detect the spectral fluctuations of the redshifted HI 21 cm signal. This signal is weaker by several orders of magnitude than... Show moreAims: The aim of the LOFAR epoch of reionization (EoR) project is to detect the spectral fluctuations of the redshifted HI 21 cm signal. This signal is weaker by several orders of magnitude than the astrophysical foreground signals and hence, in order to achieve this, very long integrations, accurate calibration for stations and ionosphere and reliable foreground removal are essential. Methods: One of the prospective observing windows for the LOFAR EoR project will be centered at the north celestial pole (NCP). We present results from observations of the NCP window using the LOFAR highband antenna (HBA) array in the frequency range 115 MHz to 163 MHz. The data were obtained in April 2011 during the commissioning phase of LOFAR. We used baselines up to about 30 km. The data was processed using a dedicated processing pipeline which is an enhanced version of the standard LOFAR processing pipeline. Results: With about 3 nights, of 6 h each, effective integration we have achieved a noise level of about 100 {$μ$}Jy/PSF in the NCP window. Close to the NCP, the noise level increases to about 180 {$μ$}Jy/PSF, mainly due to additional contamination from unsubtracted nearby sources. We estimate that in our best night, we have reached a noise level only a factor of 1.4 above the thermal limit set by the noise from our Galaxy and the receivers. Our continuum images are several times deeper than have been achieved previously using the WSRT and GMRT arrays. We derive an analytical explanation for the excess noise that we believe to be mainly due to sources at large angular separation from the NCP. We present some details of the data processing challenges and how we solved them. Conclusions: Although many LOFAR stations were, at the time of the observations, in a still poorly calibrated state we have seen no artefacts in our images which would prevent us from producing deeper images in much longer integrations on the NCP window which are about to commence. The limitations present in our current results are mainly due to sidelobe noise from the large number of distant sources, as well as errors related to station beam variations and rapid ionospheric phase fluctuations acting on bright sources. We are confident that we can improve our results with refined processing. Show less
Observations of higher-excited transitions of abundant molecules such as CO are important for determining where energy in the form of shocks is fed back into the parental envelope of forming stars... Show moreObservations of higher-excited transitions of abundant molecules such as CO are important for determining where energy in the form of shocks is fed back into the parental envelope of forming stars. The nearby prototypical and protobinary low-mass hot core, IRAS 16293-2422 (I16293) is ideal for such a study. The source was targeted with ALMA for science verification purposes in band 9, which includes CO J = 6-5 (E$_{up}$/k$_B$ ~{} 116 K), at an unprecedented spatial resolution (~{}0.''2, 25 AU). I16293 itself is composed of two sources, A and B, with a projected distance of 5''. CO J = 6-5 emission is detected throughout the region, particularly in small, arcsecond-sized hotspots, where the outflow interacts with the envelope. The observations only recover a fraction of the emission in the line wings when compared to data from single-dish telescopes, with a higher fraction of emission recovered at higher velocities. The very high angular resolution of these new data reveal that a bow shock from source A coincides, in the plane of the sky, with the position of source B. Source B, on the other hand, does not show current outflow activity. In this region, outflow entrainment takes place over large spatial scales, {gsim}100 AU, and in small discrete knots. This unique dataset shows that the combination of a high-temperature tracer (e.g., CO J = 6-5) and very high angular resolution observations is crucial for interpreting the structure of the warm inner environment of low-mass protostars. Appendices are available in electronic form at http://www.aanda.orgShow less
Cortesi, A.; Arnaboldi, M.; Coccato, L.; Merrifield, M.; Gerhard, O.; Bamford, S.; ... ; Pota, V. 2013
Context. The origins of S0 galaxies remain obscure, with various mechanisms proposed for their formation, likely depending on environment. These mechanisms would imprint different signatures in... Show moreContext. The origins of S0 galaxies remain obscure, with various mechanisms proposed for their formation, likely depending on environment. These mechanisms would imprint different signatures in the galaxies' stellar kinematics out to large radii, offering a method for distinguishing between them. Aims: We aim to study a sample of six S0 galaxies from a range of environments, and use planetary nebulae (PNe) as tracers of their stellar populations out to very large radii, to determine their kinematics in order to understand their origins. Methods: Using a special-purpose instrument, the Planetary Nebula Spectrograph, we observe and extract PNe catalogues for these six systems. Results: We show that the PNe have the same spatial distribution as the starlight, that the numbers of them are consistent with what would be expected in a comparable old stellar population in elliptical galaxies, and that their kinematics join smoothly onto those derived at smaller radii from conventional spectroscopy. Conclusions: The high-quality kinematic observations presented here form an excellent set for studying the detailed kinematics of S0 galaxies, in order to unravel their formation histories. We find that PNe are good tracers of stellar kinematics in these systems. We show that the recovered kinematics are largely dominated by rotational motion, although with significant random velocities in most cases. Full Tables 3-7 are available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/549/A115Show less
Iacobelli, M.; Haverkorn Van Rijsewijk, M.; Orrú, E.; Pizzo, R.; Anderson, J.; Beck, R.; ... ; Tasse, C. 2013
Aims: The characteristic outer scale of turbulence (i.e. the scale at which the dominant source of turbulence injects energy to the interstellar medium) and the ratio of the random to ordered... Show moreAims: The characteristic outer scale of turbulence (i.e. the scale at which the dominant source of turbulence injects energy to the interstellar medium) and the ratio of the random to ordered components of the magnetic field are key parameters to characterise magnetic turbulence in the interstellar gas, which affects the propagation of cosmic rays within the Galaxy. We provide new constraints to those two parameters. Methods: We use the LOw Frequency ARray (LOFAR) to image the diffuse continuum emission in the Fan region at (l,b) ~{} (137.0{deg}, +7.0{deg}) at 80'' { imes} 70'' resolution in the range [146, 174] MHz. We detect multi-scale fluctuations in the Galactic synchrotron emission and compute their power spectrum. Applying theoretical estimates and derivations from the literature for the first time, we derive the outer scale of turbulence and the ratio of random to ordered magnetic field from the characteristics of these fluctuations. Results: We obtain the deepest image of the Fan region to date and find diffuse continuum emission within the primary beam. The power spectrum displays a power law behaviour for scales between 100 and 8 arcmin with a slope {$α$} = -1.84 {plusmn} 0.19. We find an upper limit of ~{}20 pc for the outer scale of the magnetic interstellar turbulence toward the Fan region, which is in agreement with previous estimates in literature. We also find a variation of the ratio of random to ordered field as a function of Galactic coordinates, supporting different turbulent regimes. Conclusions: We present the first LOFAR detection and imaging of the Galactic diffuse synchrotron emission around 160 MHz from the highly polarized Fan region. The power spectrum of the foreground synchrotron fluctuations is approximately a power law with a slope {$α$} {ap} -1.84 up to angular multipoles of {lsim}1300, corresponding to an angular scale of ~{}8 arcmin. We use power spectra fluctuations from LOFAR as well as earlier GMRT and WSRT observations to constrain the outer scale of turbulence (L$_{out}$) of the Galactic synchrotron foreground, finding a range of plausible values of 10-20 pc. Then, we use this information to deduce lower limits of the ratio of ordered to random magnetic field strength. These are found to be 0.3, 0.3, and 0.5 for the LOFAR, WSRT and GMRT fields considered respectively. Both these constraints are in agreement with previous estimates. Show less
Pelupessy, F.I.; Elteren, A.K. van; Vries, N. de; McMillan, S.; Drost, N.; Portegies Zwart, S.F. 2013
We present the open source Astrophysical Multi-purpose Software Environment (AMUSE), a component library for performing astrophysical simulations involving different physical domains and scales.... Show moreWe present the open source Astrophysical Multi-purpose Software Environment (AMUSE), a component library for performing astrophysical simulations involving different physical domains and scales. It couples existing codes within a Python framework based on a communication layer using MPI. The interfaces are standardized for each domain and their implementation based on MPI guarantees that the whole framework is well-suited for distributed computation. It includes facilities for unit handling and data storage. Currently it includes codes for gravitational dynamics, stellar evolution, hydrodynamics and radiative transfer. Within each domain the interfaces to the codes are as similar as possible. We describe the design and implementation of AMUSE, as well as the main components and community codes currently supported and we discuss the code interactions facilitated by the framework. Additionally, we demonstrate how AMUSE can be used to resolve complex astrophysical problems by presenting example applications. http://www.amusecode.org The current version of the code is available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/557/A84Show less
Martinsson, T.P.K.; Verheijen, M.; Westfall, K.; Bershady, M.; Andersen, D.; Swaters, R. 2013
We present dynamically-determined rotation-curve mass decompositions of 30 spiral galaxies, which were carried out to test the maximum-disk hypothesis and to quantify properties of their dark... Show moreWe present dynamically-determined rotation-curve mass decompositions of 30 spiral galaxies, which were carried out to test the maximum-disk hypothesis and to quantify properties of their dark-matter halos. We used measured vertical velocity dispersions of the disk stars to calculate dynamical mass surface densities ({$Sigma$}$_{dyn}$). By subtracting our observed atomic and inferred molecular gas mass surface densities from {$Sigma$}$_{dyn}$, we derived the stellar mass surface densities ({$Sigma$}$_{∗}$), and thus have absolute measurements of all dominant baryonic components of the galaxies. Using K-band surface brightness profiles (I$_K$), we calculated the K-band mass-to-light ratio of the stellar disks ({Upsilon}$_{∗}$ = {$Sigma$}$_{∗}$/I$_K$) and adopted the radial mean (overline${$mls$}$) for each galaxy to extrapolate {$Sigma$}$_{∗}$ beyond the outermost kinematic measurement. The derived overline${$mls$}$ of individual galaxies are consistent with all galaxies in the sample having equal {Upsilon}$_{∗}$. We find a sample average and scatter of mlab overline${$mls$}$mrab = 0.31 {plusmn} 0.07. Rotation curves of the baryonic components were calculated from their deprojected mass surface densities. These were used with circular-speed measurements to derive the structural parameters of the dark-matter halos, modeled as either a pseudo-isothermal sphere (pISO) or a Navarro-Frenk-White (NFW) halo. In addition to our dynamically determined mass decompositions, we also performed alternative rotation-curve decompositions by adopting the traditional maximum-disk hypothesis. However, the galaxies in our sample are submaximal, such that at 2.2 disk scale lengths (h$_R$) the ratios between the baryonic and total rotation curves (F$_b$$^{2.2hR}$) are less than 0.75. We find this ratio to be nearly constant between 1-6h$_R$ within individual galaxies. We find a sample average and scatter of mlab F$_b$$^{2.2hR}$mrab = 0.57 {plusmn} 0.07, with trends of larger F$_b$$^{2.2hR}$ for more luminous and higher-surface-brightness galaxies. To enforce these being maximal, we need to scale {Upsilon}$_{∗}$ by a factor 3.6 on average. In general, the dark-matter rotation curves are marginally better fit by a pISO than by an NFW halo. For the nominal-{Upsilon}$_{∗}$ (submaximal) case, we find that the derived NFW-halo parameters have values consistent with {$Lambda$}CDM N-body simulations, suggesting that the baryonic matter in our sample of galaxies has only had a minor effect on the dark-matter distribution. In contrast, maximum-{Upsilon}$_{∗}$ decompositions yield halo-concentration parameters that are too low compared to the {$Lambda$}CDM simulations. Appendix is available in electronic form at http://www.aanda.orgShow less
The Wide Field Camera Transit Survey is a pioneer program aiming at for searching extra-solar planets in the near-infrared. The images from the survey are processed by a data reduction pipeline,... Show moreThe Wide Field Camera Transit Survey is a pioneer program aiming at for searching extra-solar planets in the near-infrared. The images from the survey are processed by a data reduction pipeline, which uses aperture photometry to construct the light curves. We produce an alternative set of light curves using the difference-imaging method for the most complete field in the survey and carry out a quantitative comparison between the photometric precision achieved with both methods. The results show that difference-photometry light curves present an important improvement for stars with J {gt} 16. We report an implementation on the box-fitting transit detection algorithm, which performs a trapezoid-fit to the folded light curve, providing more accurate results than the box-fitting model. We describe and optimize a set of selection criteria to search for transit candidates, including the V-shape parameter calculated by our detection algorithm. The optimized selection criteria are applied to the aperture photometry and difference-imaging light curves, resulting in the automatic detection of the best 200 transit candidates from a sample of ~{}475 000 sources. We carry out a detailed analysis in the 18 best detections and classify them as transiting planet and eclipsing binary candidates. We present one planet candidate orbiting a late G-type star. No planet candidate around M-stars has been found, confirming the null detection hypothesis and upper limits on the occurrence rate of short-period giant planets around M-dwarfs presented in a prior study. We extend the search for transiting planets to stars with J {le} 18, which enables us to set a stricter upper limit of 1.1%. Furthermore, we present the detection of five faint extremely-short period eclipsing binaries and three M-dwarf/M-dwarf binary candidates. The detections demonstrate the benefits of using the difference-imaging light curves, especially when going to fainter magnitudes. Show less
Context. Rotationally supported disks are critical in the star formation process. The questions of when they form and what factors influence or hinder their formation have been studied but are... Show moreContext. Rotationally supported disks are critical in the star formation process. The questions of when they form and what factors influence or hinder their formation have been studied but are largely unanswered. Observations of early-stage YSOs are needed to probe disk formation. Aims: VLA1623 is a triple non-coeval protostellar system, with a weak magnetic field perpendicular to the outflow, whose Class 0 component, VLA1623A, shows a disk-like structure in continuum with signatures of rotation in line emission. We aim to determine whether this structure is in part or in whole a rotationally supported disk, i.e. a Keplerian disk, and what its characteristics are. Methods: ALMA Cycle 0 Early Science 1.3 mm continuum and C$^{18}$O (2-1) observations in the extended configuration are presented here and used to perform an analysis of the disk-like structure using position-velocity (PV) diagrams and thin disk modeling with the addition of foreground absorption. Results: The PV diagrams of the C$^{18}$O line emission suggest the presence of a rotationally supported component with a radius of at least 50 AU. Kinematical modeling of the line emission shows that the disk out to 180 AU is actually rotationally supported, with the rotation described well by Keplerian rotation out to at least 150 AU, and the central source mass is ~{}0.2 M$_{⊙}$ for an inclination of 55{deg}. Pure infall and conserved angular momentum rotation models are excluded. Conclusions: VLA1623A, a very young Class 0 source, presents a disk with an outer radius R$_{out}$ = 180 AU with a Keplerian velocity structure out to at least 150 AU. The weak magnetic fields and recent fragmentation in this region of {$ρ$} Ophiuchus may have played a leading role in the formation of the disk. Appendices are available in electronic form at http://www.aanda.orgShow less
CO is an important component of a protoplanetary disc as it is one of the most abundant gas phase species. Furthermore, observations of CO transitions can be used as a diagnostic of the gas,... Show moreCO is an important component of a protoplanetary disc as it is one of the most abundant gas phase species. Furthermore, observations of CO transitions can be used as a diagnostic of the gas, tracing conditions in both the inner and outer disc. We present Herschel/PACS spectroscopy of a sample of 22 Herbig Ae/Be (HAEBEs) and eight T Tauri stars (TTS), covering the pure rotational CO transitions from J = 14 { arr} 13 up to J = 49 { arr} 48. CO is detected in only five HAEBEs, namely AB Aur, HD 36112, HD 97048, HD 100546, and IRS 48, and in four TTS, namely AS 205, S CrA, RU Lup, and DG Tau. The highest transition detected is J = 36 { arr} 35 with E$_{up}$ of 3669 K, seen in HD 100546 and DG Tau. We construct rotational diagrams for the discs with at least three CO detections to derive T$_{rot}$ and find average temperatures of 270 K for the HAEBEs and 485 K for the TTS. The HD 100546 star requires an extra temperature component at T$_{rot}$ ~{} 900-1000 K, suggesting a range of temperatures in its disc atmosphere, which is consistent with thermo-chemical disc models. In HAEBEs, the objects with CO detections all have flared discs in which the gas and dust are thermally decoupled. We use a small model grid to analyse our observations and find that an increased amount of flaring means higher line flux, as it increases the mass in warm gas. CO is not detected in our flat discs as the emission is below the detection limit. We find that HAEBE sources with CO detections have high L$_{UV}$ and strong PAH emission, which is again connected to the heating of the gas. In TTS, the objects with CO detections are all sources with evidence of a disc wind or outflow. For both groups of objects, sources with CO detections generally have high UV luminosity (either stellar in HAEBEs or due to accretion in TTS), but this is not a sufficient condition for the detection of the far-IR CO lines. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendices are available in electronic form at http://www.aanda.orgShow less
Yıldız, U.; Kristensen, L.; Dishoeck, E.F. van; San Jose Garcia, I.; Karska, A.; Harsono, D.S.; ... ; Hogerheijde, M.R. 2013
Context. In the deeply embedded stage of star formation, protostars start to heat and disperse their surrounding cloud cores. The evolution of these sources has traditionally been traced through... Show moreContext. In the deeply embedded stage of star formation, protostars start to heat and disperse their surrounding cloud cores. The evolution of these sources has traditionally been traced through dust continuum spectral energy distributions (SEDs), but the use of CO excitation as an evolutionary probe has not yet been explored due to the lack of high-J CO observations. Aims: The aim is to constrain the physical characteristics (excitation, kinematics, column density) of the warm gas in low-mass protostellar envelopes using spectrally resolved Herschel data of CO and compare those with the colder gas traced by lower excitation lines. Methods: Herschel-HIFI observations of high-J lines of $^{12}$CO, $^{13}$CO, and C$^{18}$O (up to J$_u$ = 10, E$_u$ up to 300 K) are presented toward 26 deeply embedded low-mass Class 0 and Class I young stellar objects, obtained as part of the Water In Star-forming regions with Herschel (WISH) key program. This is the first large spectrally resolved high-J CO survey conducted for these types of sources. Complementary lower J CO maps were observed using ground-based telescopes, such as the JCMT and APEX and convolved to matching beam sizes. Results: The $^{12}$CO 10-9 line is detected for all objects and can generally be decomposed into a narrow and a broad component owing to the quiescent envelope and entrained outflow material, respectively. The $^{12}$CO excitation temperature increases with velocity from ~{}60 K up to ~{}130 K. The median excitation temperatures for $^{12}$CO, $^{13}$CO, and C$^{18}$O derived from single-temperature fits to the J$_u$ = 2-10 integrated intensities are ~{}70 K, 48 K and 37 K, respectively, with no significant difference between Class 0 and Class I sources and no trend with M$_{env}$ or L$_{bol}$. Thus, in contrast to the continuum SEDs, the spectral line energy distributions (SLEDs) do not show any evolution during the embedded stage. In contrast, the integrated line intensities of all CO isotopologs show a clear decrease with evolutionary stage as the envelope is dispersed. Models of the collapse and evolution of protostellar envelopes reproduce the C$^{18}$O results well, but underproduce the $^{13}$CO and $^{12}$CO excitation temperatures, due to lack of UV heating and outflow components in those models. The H$_{2}$O 1$_{10}$ - 1$_{01}$/CO 10-9 intensity ratio does not change significantly with velocity, in contrast to the H$_{2}$O/CO 3-2 ratio, indicating that CO 10-9 is the lowest transition for which the line wings probe the same warm shocked gas as H$_{2}$O. Modeling of the full suite of C$^{18}$O lines indicates an abundance profile for Class 0 sources that is consistent with a freeze-out zone below 25 K and evaporation at higher temperatures, but with some fraction of the CO transformed into other species in the cold phase. In contrast, the observations for two Class I sources in Ophiuchus are consistent with a constant high CO abundance profile. Conclusions: The velocity resolved line profiles trace the evolution from the Class 0 to the Class I phase through decreasing line intensities, less prominent outflow wings, and increasing average CO abundances. However, the CO excitation temperature stays nearly constant. The multiple components found here indicate that the analysis of spectrally unresolved data, such as provided by SPIRE and PACS, must be done with caution. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendices C and D are available in electronic form at http://www.aanda.orgShow less
Context. Understanding the physical phenomena involved in the earlierst stages of protostellar evolution requires knowledge of the heating and cooling processes that occur in the surroundings of a... Show moreContext. Understanding the physical phenomena involved in the earlierst stages of protostellar evolution requires knowledge of the heating and cooling processes that occur in the surroundings of a young stellar object. Spatially resolved information from its constituent gas and dust provides the necessary constraints to distinguish between different theories of accretion energy dissipation into the envelope. Aims. Our aims are to quantify the far-infrared line emission from low-mass protostars and the contribution of different atomic and molecular species to the gas cooling budget, to determine the spatial extent of the emission, and to investigate the underlying excitation conditions. Analysis of the line cooling will help us characterize the evolution of the relevant physical processes as the protostar ages. Methods. Far-infrared Herschel-PACS spectra of 18 low-mass protostars of various luminosities and evolutionary stages are studied in the context of the WISH key program. For most targets, the spectra include many wavelength intervals selected to cover specific CO, H$_{2}$O, OH, and atomic lines. For four targets the spectra span the entire 55-200 {$μ$}m region. The PACS field-of-view covers ~{}47'' with the resolution of 9.4''. Results. Most of the protostars in our sample show strong atomic and molecular far-infrared emission. Water is detected in 17 out of 18 objects (except TMC1A), including 5 Class I sources. The high-excitation H$_{2}$O 8$_{18}$-7$_{07}$ 63.3 {$μ$}m line (E$_u$/k$_B$ = 1071 K) is detected in 7 sources. CO transitions from J = 14-13 up to J = 49 - 48 are found and show two distinct temperature components on Boltzmann diagrams with rotational temperatures of ~{}350 K and ~{}700 K. H$_{2}$O has typical excitation temperatures of ~{}150 K. Emission from both Class 0 and I sources is usually spatially extended along the outflow direction but with a pattern that depends on the species and the transition. In the extended sources, emission is stronger off source and extended on {amp}{ge}10,000 AU scales; in the compact sample, more than half of the flux originates within 1000 AU of the protostar. The H$_{2}$O line fluxes correlate strongly with those of the high-J CO lines, both for the full array and for the central position, as well as with the bolometric luminosity and envelope mass. They correlate less strongly with OH fluxes and not with [O I] fluxes. In contrast, [O I] and OH often peak together at the central position. Conclusions. The PACS data probe at least two physical components. The H$_{2}$O and CO emission very likely arises in non-dissociative (irradiated) shocks along the outflow walls with a range of pre-shock densities. Some OH is also associated with this component, most likely resulting from H$_{2}$O photodissociation. UV-heated gas contributes only a minor fraction to the CO emission observed by PACS, based on the strong correlation between the shock-dominated CO 24-23 line and the CO 14-13 line. [O I] and some of the OH emission probe dissociative shocks in the inner envelope. The total far-infrared cooling is dominated by H$_{2}$O and CO, with the fraction contributed by [O I] increasing for Class I sources. Consistent with previous studies, the ratio of total far-infrared line emission over bolometric luminosity decreases with the evolutionary state. Appendices A-J are available in electronic form at http://www.aanda.orgShow less
Tak, F.; Chavarría, L.; Herpin, F.; Wyrowski, F.; Walmsley, C.; Dishoeck, E.F. van; ... ; Tafalla, M. 2013
Context. Water is a key constituent of star-forming matter, but the origin of its line emission and absorption during high-mass star formation is not well understood. Aims: We study the velocity... Show moreContext. Water is a key constituent of star-forming matter, but the origin of its line emission and absorption during high-mass star formation is not well understood. Aims: We study the velocity profiles of low-excitation H$_{2}$O lines toward 19 high-mass star-forming regions and search for trends with luminosity, mass, and evolutionary stage. Methods: We decompose high-resolution Herschel-HIFI line spectra near 990, 1110 and 1670 GHz into three distinct physical components. Dense cores (protostellar envelopes) are usually seen as narrow absorptions in the H$_{2}$O 1113 and 1669 GHz ground-state lines, the H$_{2}$O 987 GHz excited-state line, and the H$_{2}$$^{18}$O 1102 GHz ground-state line. In a few sources, the envelopes appear in emission in some or all studied lines, indicating higher temperatures or densities. Broader features due to outflows are usually seen in absorption in the H$_{2}$O 1113 and 1669 GHz lines, in 987 GHz emission, and not seen in H$_{2}$$^{18}$O, indicating a lower column density and a higher excitation temperature than the envelope component. A few outflows are detected in H$_{2}$$^{18}$O, indicating higher column densities of shocked gas. In addition, the H$_{2}$O 1113 and 1669 GHz spectra show narrow absorptions by foreground clouds along the line of sight. The lack of corresponding features in the 987 GHz and H$_{2}$$^{18}$O lines indicates a low column density and a low excitation temperature for these clouds, although their derived H$_{2}$O ortho/para ratios are close to 3. Results: The intensity of the ground state lines of H$_{2}$O at 1113 and 1669 GHz does not show significant trends with source luminosity, envelope mass, or evolutionary state. In contrast, the flux in the excited-state 987 GHz line appears correlated with luminosity and the H$_{2}$$^{18}$O line flux appears correlated with the envelope mass. Furthermore, appearance of the envelope in absorption in the 987 GHz and H$_{2}$$^{18}$O lines seems to be a sign of an early evolutionary stage, as probed by the mid-infrared brightness and the L$_{bol}$/M$_{env}$ ratio of the source. Conclusions: The ground state transitions of H$_{2}$O trace the outer parts of the envelopes, so that the effects of star formation are mostly noticeable in the outflow wings. These lines are heavily affected by absorption, so that line ratios of H$_{2}$O involving the ground states must be treated with caution, especially if multiple clouds are superposed as in the extragalactic case. The isotopic H$_{2}$$^{18}$O line appears to trace the mass of the protostellar envelope, indicating that the average H$_{2}$O abundance in high-mass protostellar envelopes does not change much with time. The excited state line at 987 GHz increases in flux with luminosity and appears to be a good tracer of the mean weighted dust temperature of the source, which may explain why it is readily seen in distant galaxies. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendices are available in electronic form at http://www.aanda.orgShow less
Context. Star and planet formation theories predict an evolution in the density, temperature, and velocity structure as the envelope collapses and forms an accretion disk. While continuum emission... Show moreContext. Star and planet formation theories predict an evolution in the density, temperature, and velocity structure as the envelope collapses and forms an accretion disk. While continuum emission can trace the dust evolution, spectrally resolved molecular lines are needed to determine the physical structure and collapse dynamics. Aims: The aim of this work is to model the evolution of the molecular excitation, line profiles, and related observables during low-mass star formation. Specifically, the signatures of disks during the deeply embedded stage (M$_{env}$ {gt} M$_{⋆}$) are investigated. Methods: The semi-analytic 2D axisymmetric model of Visser and collaborators has been used to describe the evolution of the density, stellar mass, and luminosity from the pre-stellar to the T-Tauri phase. A full radiative transfer calculation is carried out to accurately determine the time-dependent dust temperatures. The time-dependent CO abundance is obtained from the adsorption and thermal desorption chemistry. Non-LTE near-IR (NIR), far-IR (FIR), and submm lines of CO have been simulated at a number of time steps. Results: In single dish (10-20'' beams), the dynamics during the collapse are best probed through highly excited $^{13}$CO and C$^{18}$O lines, which are significantly broadened by the infall process. In contrast to the dust temperature, the CO excitation temperature derived from submm/FIR data does not vary during the protostellar evolution, consistent with C$^{18}$O observations obtained with Herschel and from ground-based telescopes. The NIR spectra provide complementary information to the submm lines by probing not only the cold outer envelope but also the warm inner region. The NIR high-J ({ge}8) absorption lines are particularly sensitive to the physical structure of the inner few AU, which does show evolution. The models indicate that observations of $^{13}$CO and C$^{18}$O low-J submm lines within a {le}1{Prime} (at 140 pc) beam are well suited to probe embedded disks in Stage I (M$_{env}$ {lt} M$_{⋆}$) sources, consistent with recent interferometric observations. High signal-to-noise ratio subarcsec resolution data with ALMA are needed to detect the presence of small rotationally supported disks during the Stage 0 phase and various diagnostics are discussed. The combination of spatially and spectrally resolved lines with ALMA and at NIR is a powerful method to probe the inner envelope and disk formation process during the embedded phase. Appendices are available in electronic form at http://www.aanda.orgShow less
