arXiv subjects

M. Tafalla

Publications and source records attributed to M. Tafalla.

At least 91 records · Page 5Linked to original sources

arXiv2010

Herschel-HIFI detections of hydrides towards AFGL 2591 (Envelope emission versus tenuous cloud absorption)

The Heterodyne Instrument for the Far Infrared (HIFI) onboard the Herschel Space Observatory allows the first observations of light diatomic molecules at high spectral resolution and in multiple transitions. Here, we report deep integrations using HIFI in different lines of hydrides towards the high-mass star forming region AFGL 2591. Detected are CH, CH+, NH, OH+, H2O+, while NH+ and SH+ have not been detected. All molecules except for CH and CH+ are seen in absorption with low excitation temperatures and at velocities different from the systemic velocity of the protostellar envelope. Surprisingly, the CH(JF,P = 3/2_2,- - 1/2_1,+) and CH+(J = 1 - 0, J = 2 - 1) lines are detected in emission at the systemic velocity. We can assign the absorption features to a foreground cloud and an outflow lobe, while the CH and CH+ emission stems from the envelope. The observed abundance and excitation of CH and CH+ can be explained in the scenario of FUV irradiated outflow walls, where a cavity etched out by the outflow allows protostellar FUV photons to irradiate and heat the envelope at larger distances driving the chemical reactions that produce these molecules.

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arXiv2010

Water in low-mass star-forming regions with Herschel: HIFI spectroscopy of NGC1333

'Water In Star-forming regions with Herschel' (WISH) is a key programme dedicated to studying the role of water and related species during the star-formation process and constraining the physical and chemical properties of young stellar objects. The Heterodyne Instrument for the Far-Infrared (HIFI) on the Herschel Space Observatory observed three deeply embedded protostars in the low-mass star-forming region NGC1333 in several H2-16O, H2-18O, and CO transitions. Line profiles are resolved for five H16O transitions in each source, revealing them to be surprisingly complex. The line profiles are decomposed into broad (>20 km/s), medium-broad (~5-10 km/s), and narrow (<5 km/s) components. The H2-18O emission is only detected in broad 1_10-1_01 lines (>20 km/s), indicating that its physical origin is the same as for the broad H2-16O component. In one of the sources, IRAS4A, an inverse P Cygni profile is observed, a clear sign of infall in the envelope. From the line profiles alone, it is clear that the bulk of emission arises from shocks, both on small (<1000 AU) and large scales along the outflow cavity walls (~10 000 AU). The H2O line profiles are compared to CO line profiles to constrain the H2O abundance as a function of velocity within these shocked regions. The H2O/CO abundance ratios are measured to be in the range of ~0.1-1, corresponding to H2O abundances of ~10-5-10-4 with respect to H2. Approximately 5-10% of the gas is hot enough for all oxygen to be driven into water in warm post-shock gas, mostly at high velocities.

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arXiv2010

Sensitive limits on the abundance of cold water vapor in the DM Tau protoplanetary disk

We performed a sensitive search for the ground-state emission lines of ortho- and para-water vapor in the DM Tau protoplanetary disk using the Herschel/HIFI instrument. No strong lines are detected down to 3sigma levels in 0.5 km/s channels of 4.2 mK for the 1_{10}--1_{01} line and 12.6 mK for the 1_{11}--0_{00} line. We report a very tentative detection, however, of the 1_{10}--1_{01} line in the Wide Band Spectrometer, with a strength of T_{mb}=2.7 mK, a width of 5.6 km/s and an integrated intensity of 16.0 mK km/s. The latter constitutes a 6sigma detection. Regardless of the reality of this tentative detection, model calculations indicate that our sensitive limits on the line strengths preclude efficient desorption of water in the UV illuminated regions of the disk. We hypothesize that more than 95-99% of the water ice is locked up in coagulated grains that have settled to the midplane.

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arXiv2010

Herschel observations of the hydroxyl radical (OH) in young stellar objects

Water in Star-forming regions with Herschel (WISH) is a Herschel Key Program investigating the water chemistry in young stellar objects (YSOs) during protostellar evolution. Hydroxyl (OH) is one of the reactants in the chemical network most closely linked to the formation and destruction of H2O. High-temperature chemistry connects OH and H2O through the OH + H2 <-> H2O + H reactions. Formation of H2O from OH is efficient in the high-temperature regime found in shocks and the innermost part of protostellar envelopes. Moreover, in the presence of UV photons, OH can be produced from the photo-dissociation of H2O. High-resolution spectroscopy of the OH 163.12 micron triplet towards HH 46 and NGC 1333 IRAS 2A was carried out with the Heterodyne Instrument for the Far Infrared (HIFI) on board Herschel. The low- and intermediate-mass YSOs HH 46, TMR 1, IRAS 15398-3359, DK Cha, NGC 7129 FIRS 2, and NGC 1333 IRAS 2A were observed with the Photodetector Array Camera and Spectrometer (PACS) in four transitions of OH and two [OI] lines. The OH transitions at 79, 84, 119, and 163 micron and [OI] emission at 63 and 145 micron were detected with PACS towards the class I low-mass YSOs as well as the intermediate-mass and class I Herbig Ae sources. No OH emission was detected from the class 0 YSO NGC 1333 IRAS 2A, though the 119 micron was detected in absorption. With HIFI, the 163.12 micron was not detected from HH 46 and only tentatively detected from NGC 1333 IRAS 2A. The combination of the PACS and HIFI results for HH 46 constrains the line width (FWHM > 11 km/s) and indicates that the OH emission likely originates from shocked gas. This scenario is supported by trends of the OH flux increasing with the [OI] flux and the bolometric luminosity. Similar OH line ratios for most sources suggest that OH has comparable excitation temperatures despite the different physical properties of the sources.

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arXiv2010

Water vapor toward starless cores: the Herschel view

SWAS and Odin provided stringent upper limits on the gas phase water abundance of dark clouds (x(H2O) < 7x10^-9). We investigate the chemistry of water vapor in starless cores beyond the previous upper limits using the highly improved angular resolution and sensitivity of Herschel and measure the abundance of water vapor during evolutionary stages just preceding star formation. High spectral resolution observations of the fundamental ortho water (o-H2O) transition (557 GHz) were carried out with Herschel HIFI toward two starless cores: B68, a Bok globule, and L1544, a prestellar core embedded in the Taurus molecular cloud complex. The rms in the brightness temperature measured for the B68 and L1544 spectra is 2.0 and 2.2 mK, respectively, in a velocity bin of 0.59 km s^-1. The continuum level is 3.5+/-0.2 mK in B68 and 11.4+/-0.4 mK in L1544. No significant feature is detected in B68 and the 3 sigma upper limit is consistent with a column density of o-H2O N(o-H2O) < 2.5x10^13 cm^-2, or a fractional abundance x(o-H2O) < 1.3x10^-9, more than an order of magnitude lower than the SWAS upper limit on this source. The L1544 spectrum shows an absorption feature at a 5 sigma level from which we obtain the first value of the o-H2O column density ever measured in dark clouds: N(o-H2O) = (8+/-4)x10^12 cm^-2. The corresponding fractional abundance is x(o-H2O) ~ 5x10^-9 at radii > 7000 AU and ~2x10^-10 toward the center. The radiative transfer analysis shows that this is consistent with a x(o-H2O) profile peaking at ~10^-8, 0.1 pc away from the core center, where both freeze-out and photodissociation are negligible. Herschel has provided the first measurement of water vapor in dark regions. Prestellar cores such as L1544 (with their high central densities, strong continuum, and large envelopes) are very promising tools to finally shed light on the solid/vapor balance of water in molecular clouds.

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arXiv2010

Chemical study of intermediate-mass (IM) Class 0 protostars. CO depletion and N2H+ deuteration

We are carrying out a physical and chemical study of the protostellar envelopes in a representative sample of IM Class 0 protostars. In our first paper (Crimier et al. 2010), we determined the physical structure (density-temperature radial profiles) of the protostellar envelopes. Here, we study the CO depletion and N2H+ deuteration. We observed the millimeter lines of C18O, C17O, N2H+ and N2D+ toward the protostars using the IRAM 30m telescope. Based on these observations, we derived the C18O, N2H+ and N2D+ radial abundance profiles across their envelopes using a radiative transfer code. In addition, we modeled the chemistry of the protostellar envelopes. All the C18O 1-0 maps are well fit assuming that the C18O abundance decreases inwards within the protostellar envelope until the gas and dust reach the CO evaporation temperature, 20-25K, where the CO is released back to the gas phase. The N2H+ deuterium fractionation in Class 0 IMs is [N2D+]/[N2H+]=0.005-0.014, two orders of magnitude higher than the elemental [D/H] value in the interstellar medium, but a factor of 10 lower than in pre-stellar clumps. Chemical models account for the C18O and N2H+ observations if we assume the CO abundance is 2 times lower than the canonical value in the inner envelope. This could be the consequence of the CO being converted into CH3OH on the grain surfaces prior to the evaporation and/or the photodissociation of CO by the stellar UV radiation. The deuterium fractionation is not fitted by chemical models. This discrepancy is very likely caused by the simplicity of our model that assumes spherical geometry and neglects important phenomena like the effect of bipolar outflows and UV radiation from the star. More important, the deuterium fractionation is dependent on the ortho-to-para H2 ratio, which is not likely to reach the steady-state value in the dynamical time scales of these protostars.

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arXiv2010

Herschel-PACS spectroscopy of the intermediate mass protostar NGC7129 FIRS 2

Aims: We present preliminary results of the first Herschel spectroscopic observations of NGC7129 FIRS2, an intermediate mass star-forming region. We attempt to interpret the observations in the framework of an in-falling spherical envelope. Methods: The PACS instrument was used in line spectroscopy mode (R=1000-5000) with 15 spectral bands between 63 and 185 microns. This provided good detections of 26 spectral lines seen in emission, including lines of H2O, CO, OH, O I, and C II. Results: Most of the detected lines, particularly those of H2O and CO, are substantially stronger than predicted by the spherical envelope models, typically by several orders of magnitude. In this paper we focus on what can be learned from the detected CO emission lines. Conclusions: It is unlikely that the much stronger than expected line emission arises in the (spherical) envelope of the YSO. The region hot enough to produce such high excitation lines within such an envelope is too small to produce the amount of emission observed. Virtually all of this high excitation emission must arise in structures such as as along the walls of the outflow cavity with the emission produced by a combination of UV photon heating and/or non-dissociative shocks.

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arXiv2010

Water cooling of shocks in protostellar outflows: Herschel-PACS map of L1157

In the framework of the Water in Star-forming regions with Herschel (WISH) key program, maps in water lines of several outflows from young stars are being obtained, to study the water production in shocks and its role in the outflow cooling. This paper reports the first results of this program, presenting a PACS map of the o-H2O 179 um transition obtained toward the young outflow L1157. The 179 um map is compared with those of other important shock tracers, and with previous single-pointing ISO, SWAS, and Odin water observations of the same source that allow us to constrain the water abundance and total cooling. Strong H2O peaks are localized on both shocked emission knots and the central source position. The H2O 179 um emission is spatially correlated with emission from H2 rotational lines, excited in shocks leading to a significant enhancement of the water abundance. Water emission peaks along the outflow also correlate with peaks of other shock-produced molecular species, such as SiO and NH3. A strong H2O peak is also observed at the location of the proto-star, where none of the other molecules have significant emission. The absolute 179 um intensity and its intensity ratio to the H2O 557 GHz line previously observed with Odin/SWAS indicate that the water emission originates in warm compact clumps, spatially unresolved by PACS, having a H2O abundance of the order of 10^-4. This testifies that the clumps have been heated for a time long enough to allow the conversion of almost all the available gas-phase oxygen into water. The total water cooling is ~10^-1 Lo, about 40% of the cooling due to H2 and 23% of the total energy released in shocks along the L1157 outflow.

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arXiv2010

Origin of the hot gas in low-mass protostars: Herschel-PACS spectroscopy of HH 46

'Water in Star-forming regions with Herschel' (WISH) is a Herschel Key Programme aimed at understanding the physical and chemical structure of young stellar objects (YSOs) with a focus on water and related species. The low-mass protostar HH 46 was observed with the Photodetector Array Camera and Spectrometer (PACS) on the Herschel Space Observatory to measure emission in H2O, CO, OH, [OI], and [CII] lines located between 63 and 186 um. The excitation and spatial distribution of emission can disentangle the different heating mechanisms of YSOs, with better spatial resolution and sensitivity than previously possible. Far-IR line emission is detected at the position of the protostar and along the outflow axis. The OH emission is concentrated at the central position, CO emission is bright at the central position and along the outflow, and H2O emission is concentrated in the outflow. In addition, [OI] emission is seen in low-velocity gas, assumed to be related to the envelope, and is also seen shifted up to 170 km/s in both the red- and blue-shifted jets. Envelope models are constructed based on previous observational constraints. They indicate that passive heating of a spherical envelope by the protostellar luminosity cannot explain the high-excitation molecular gas detected with PACS, including CO lines with upper levels at >2500 K above the ground state. Instead, warm CO and H2O emission is probably produced in the walls of an outflow-carved cavity in the envelope, which are heated by UV photons and non-dissociative C-type shocks. The bright OH and [OI] emission is attributed to J-type shocks in dense gas close to the protostar. In the scenario described here, the combined cooling by far-IR lines within the central spatial pixel is estimated to be 2 \times 10-2 L_sun, with 60-80% attributed to J- and C-type shocks produced by interactions between the jet and the envelope.

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arXiv2010

Physical structure of the envelopes of intermediate-mass protostars

Context: Intermediate mass protostars provide a bridge between low- and high-mass protostars. Furthermore, they are an important component of the UV interstellar radiation field. Despite their relevance, little is known about their formation process. Aims: We present a systematic study of the physical structure of five intermediate mass, candidate Class 0 protostars. Our two goals are to shed light on the first phase of intermediate mass star formation and to compare these protostars with low- and high-mass sources. Methods: We derived the dust and gas temperature and density profiles of the sample. We analysed all existing continuum data on each source and modelled the resulting SED with the 1D radiative transfer code DUSTY. The gas temperature was then predicted by means of a modified version of the code CHT96. Results: We found that the density profiles of five out of six studied intermediate mass envelopes are consistent with the predictions of the "inside-out" collapse theory.We compared several physical parameters, like the power law index of the density profile, the size, the mass, the average density, the density at 1000 AU and the density at 10 K of the envelopes of low-, intermediate, and high-mass protostars. When considering these various physical parameters, the transition between the three groups appears smooth, suggesting that the formation processes and triggers do not substantially differ.

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arXiv2009

CCH in prestellar cores

We study the abundance of CCH in prestellar cores both because of its role in the chemistry and because it is a potential probe of the magnetic field. We also consider the non-LTE behaviour of the N=1-0 and N=2-1 transitions of CCH and improve current estimates of the spectroscopic constants of CCH. We used the IRAM 30m radiotelescope to map the N=1-0 and N=2-1 transitions of CCH towards the prestellar cores L1498 and CB246. Towards CB246, we also mapped the 1.3 mm dust emission, the J=1-0 transition of N2H+ and the J=2-1 transition of C18O. We used a Monte Carlo radiative transfer program to analyse the CCH observations of L1498. We derived the distribution of CCH column densities and compared with the H2 column densities inferred from dust emission. We find that while non-LTE intensity ratios of different components of the N=1-0 and N=2-1 lines are present, they are of minor importance and do not impede CCH column density determinations based upon LTE analysis. Moreover, the comparison of our Monte-Carlo calculations with observations suggest that the non-LTE deviations can be qualitatively understood. For L1498, our observations in conjunction with the Monte Carlo code imply a CCH depletion hole of radius 9 x 10^{16} cm similar to that found for other C-containing species. We briefly discuss the significance of the observed CCH abundance distribution. Finally, we used our observations to provide improved estimates for the rest frequencies of all six components of the CCH(1-0) line and seven components of CCH(2-1). Based on these results, we compute improved spectroscopic constants for CCH. We also give a brief discussion of the prospects for measuring magnetic field strengths using CCH.

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arXiv2009

The extremely collimated bipolar H_2O jet from the NGC 1333-IRAS 4B protostar

We have performed observations of water maser emission towards a sample of low-mass protostars, in order to investigate the properties of jets associated with the earliest stages of star formation and their interaction with the surrounding medium. The main aim is to measure the absolute positions and proper motions of the H_2O spots in order to investigate the kinematics of the region from where the jet is launched. We imaged the protostars in the nearby region NGC 1333-IRAS 4 in the water maser line at 22.2 GHz by using the VLBA in phase-reference mode at the milliarcsecond scale over four epochs, spaced by one month to measure proper motions. Two protostars (A2 and B) were detected in a highly variable H_2O maser emission, with an active phase shorter than four weeks. The H_2O maps allow us to trace the fast jet driven by the B protostar: we observed both the red- and blue-shifted lobes very close to the protostar, =< 35 AU, moving away with projected velocities of ~10-50 km/s. The comparison with the molecular outflow observed at larger scale suggests a jet precession with a 18'/yr rate. By measuring the positional spread of the H_2O spots we estimate a jet width of ~2 AU at a distance of ~12 AU from the driving protostar.

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arXiv2008

A highly collimated jet from the low mass proto-star NGC1333 IRAS 4B

We imaged the protostars of the nearby region NGC1333 IRAS 4 in the water maser line at 22.2 GHz by using the VLBA in phase referencing at milliarcsecond scale over four epochs spaced by one month to measure proper motions. We measure the absolute positions and proper motions of the H2O spots to investigate the kinematics of the region from where the jet is launched. Two protostars (A2 and B) have been detected in a highly variable H2O maser emission, with an active phase shorter than four weeks. A 70 AU chain of several maser spots, very well aligned, has been observed close to the B protostar. The apparent proper motions have been derived, finding that the H2O spots are moving along the N-NW direction with projected velocities between 10 and 50 km/s. We conclude that in IRAS 4B, water maser trace a highly collimated bipolar jet clearly associated with the protostar.

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arXiv2008

Shells, jets, and internal working surfaces in the molecular outflow from IRAS 04166+2706

Context: IRAS 04166+2706 in Taurus is one of the most nearby young stellar objects whose molecular outflow contains a highly collimated fast component. Methods: We have observed the IRAS 04166+2706 outflow with the IRAM Plateau de Bure interferometer in CO(J=2-1) and SiO(J=2-1) achieving angular resolutions between 2'' and 4''. To improve the quality of the CO(2-1) images, we have added single dish data to the interferometer visibilities. Results: The outflow consists of two distinct components. At velocities <10 km/s, the gas forms two opposed, approximately conical shells that have the YSO at their vertex. These shells coincide with the walls of evacuated cavities and seem to result from the acceleration of the ambient gas by a wide-angle wind. At velocities >30 km/s, the gas forms two opposed jets that travel along the center of the cavities and whose emission is dominated by a symmetric collection of at least 7 pairs of peaks. The velocity field of this component presents a sawtooth pattern with the gas in the tail of each peak moving faster than the gas in the head. This pattern, together with a systematic widening of the peaks with distance to the central source, is consistent with the emission arising from internal working surfaces traveling along the jet and resulting from variations in the velocity field of ejection. We interpret this component as the true protostellar wind, and we find its composition consistent with a chemical model of such type of wind. Conclusions: Our results support outflow wind models that have simultaneously wide-angle and narrow components, and suggest that the EHV peaks seen in a number of outflows consist of internally-shocked wind material.

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arXiv2007

The IC1396N proto-cluster at a scale of 250 AU

We investigate the mm-morphology of IC1396N with unprecedented spatial resolution to analyze its dust and molecular gas properties, and draw comparisons with objects of similar mass. We have carried out sensitive observations in the most extended configurations of the IRAM Plateau de Bure interferometer, to map the thermal dust emission at 3.3 and 1.3mm, and the emission from the $J$=13$_k\to12_k$ hyperfine transitions of methyl cyanide (CH$_3$CN). We unveil the existence of a sub-cluster of hot cores in IC1396N, distributed in a direction perpendicular to the emanating outflow. The cores are embedded in a common envelope of extended and diffuse dust emission. We find striking differences in the dust properties of the cores ($β\simeq$ 0) and the surrounding envelope ($β\simeq$ 1), very likely testifying to differences in the formation and processing of dust material. The CH$_3$CN emission peaks towards the most massive hot core and is marginally extended in the outflow direction.

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arXiv2007

Studies of Dense Cores with ALMA

Dense cores are the simplest star-forming sites that we know, but despite their simplicity, they still hold a number of mysteries that limit our understanding of how solar-type stars form. ALMA promises to revolutionize our knowledge of every stage in the life of a core, from the pre-stellar phase to the final disruption by the newly born star. This contribution presents a brief review of the evolution of dense cores and illustrates particular questions that will greatly benefit from the increase in resolution and sensitivity expected from ALMA

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arXiv2006

Warm SiO gas in molecular bullets associated with protostellar outflows

In this paper we present the first SiO multiline analysis (from J=2-1 to J=11-10) of the molecular bullets along the outflows of the Class 0 sources L1448-mm and L1157-mm, obtained through observations with IRAM and JCMT. We have computed the main physical parameters in each bullet and compared them with other tracers of warm and dense gas and with models for the SiO excitation in shocks. We find that the bullets close to L1448--mm, associated with high velocity gas, have higher excitation conditions (n(H2) ~ 10^{6} cm^{-3}, T > 500 K) with respect to the L1157 bullets (n(H2) ~1-5 10^{5} cm^{-3}, T ~ 100-300 K). In both the sources, there is a clear evidence of the presence of velocity components having different excitation conditions, with the denser and/or warmer gas associated with the gas at the higher speed. In L1448 the bulk of the emission is due to the high-excitation and high velocity gas, while in L1157 most of the emission comes from the low excitation gas at ambient velocity. The observed velocity-averaged line ratios are well reproduced by shocks with speeds v_s larger than ~ 30 km/s and densities ~ 10^{5} - 10^{6} cm^{-3}. Plane-parallel shock models, however, fail to predict all the observed line profiles and in particular the very similar profiles shown by both low and high excitation lines. The overall observations support the idea that the L1157 clumps are shock interaction events older than the L1448 bullets close to the driving source. In the latter objects, the velocity structure and the variations of physical parameters with the velocity resemble very closely those found in optical/IR jets near the protostar, suggesting that similar launching and excitation mechanisms are also at the origin of collimated jets seen at millimetre wavelengths.

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arXiv2006

Star formation in the vicinity of the IC 348 cluster

Aims. We present molecular line observations of the southwestern part of the IC 348 young cluster, and we use them together with NIR and mm continuum data to determine the distribution of dense gas, search for molecular outflows, and analyze the ongoing star formation activity in the region. Methods. Our molecular line data consists of C18O(1--0) and N2H+(1--0) maps obtained with the FCRAO telescope at a resolution of about 50'' and CO(2--1) data obtained with the IRAM 30m telescope at a resolution of 11''. Results. The dense gas southwest of IC 348 is concentrated in two groups of dense cores, each of them with a few solar masses of material and indications of CO depletion at high density. One of the core groups is actively forming stars, while the other seems starless. There is evidence for at least three bipolar molecular outflows in the region, two of them powered by previously identified Class 0 sources, while the other one is powered by a still not well characterized low-luminosity object. The ongoing star formation activity is producing a small stellar subgroup in the cluster. Using the observed core characteristics and the star formation rate in the cluster we propose that that similar episodes of stellar birth may have produced the subclustering observed in the halo of IC 348.

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