Á. Sánchez-Monge

Deuteration as an evolutionary tracer in massive-star formation

Context. Theory predicts, and observations confirm, that the column density ratio of a molecule containing D to its counterpart containing H can be used as an evolutionary tracer in the low-mass star formation process. Aims. Since it remains unclear if the high-mass star formation process is a scaled-up version of the low-mass one, we investigated whether the relation between deuteration and evolution can be applied to the high-mass regime. Methods. With the IRAM-30 m telescope, we observed rotational transitions of N 2 D + and N 2 H + and derived the deuterated fraction in 27 cores within massive star-forming regions understood to represent different evolutionary stages of the massive-star formation process. Results. The abundance of N 2 D + is higher at the pre-stellar/cluster stage, then drops during the formation of the protostellar object(s) as in the low-mass regime, remaining relatively constant during the ultra-compact HII region phase. The objects with the highest fractional abundance of N 2 D + are starless cores with properties very similar to typical pre-stellar cores of lower mass. The abundance of N 2 D + is lower in objects with higher gas temperatures as in the low-mass case but does not seem to depend on gas turbulence. Conclusions. Our results indicate that the N 2 D + -to-N 2 H + column density ratio can be used as an evolutionary indicator in both low-and high-mass star formation, and that the physical conditions influencing the abundance of deuterated species likely evolve similarly during the processes that lead to the formation of both low- and high-mass stars.

Ubiquitous argonium (ArH+) in the diffuse interstellar medium: A molecular tracer of almost purely atomic gas

Aims. We describe the assignment of a previously unidentified interstellar absorption line to ArH + and discuss its relevance in the context of hydride absorption in di use gas with a low H2 fraction. The confidence of the assignment to ArH + is discussed, and the column densities are determined toward several lines of sight. The results are then discussed in the framework of chemical models, with the aim of explaining the observed column densities. Methods. We fitted the spectral lines with multiple velocity components, and determined column densities from the line-to-continuum ratio. The column densities of ArH + were compared to those of other species, tracing interstellar medium (ISM) components with di erent H2 abundances. We constructed chemical models that take UV radiation and cosmic ray ionization into account.

Different evolutionary stages in massive star formation - Centimeter continuum and H2O maser emission with ATCA

We present ATCA observations of the H2O maser line and radio continuum at 18.0GHz and 22.8GHz, toward a sample of 192 massive star forming regions containing several clumps already imaged at 1.2mm. The main aim of this study is to investigate the water maser and centimeter continuum emission (likely tracing thermal free-free emission) in sources at different evolutionary stages, using the evolutionary classifications proposed by Palla et al (1991) and Molinari et al (2008). We used the recently comissioned CABB backend at ATCA obtaining images with 20arcsec resolution in the 1.3cm continuum and H2O maser emission, in all targets. For the evolutionary analysis of the sources we used the millimeter continuum emission from Beltran et al (2006) and the infrared emission from the MSX Point Source Catalogue. We detect centimeter continuum emission in 88% of the observed fields with a typical rms noise level of 0.45mJy/beam. Most of the fields show a single radio continuum source, while in 20% of them we identify multiple components. A total of 214 centimeter continuum sources have been identified, likely tracing optically thin HII regions, with physical parameters typical of both extended and compact HII regions. Water maser emission was detected in 41% of the regions, resulting in a total of 85 distinct components. The low angular (20arcsec) and spectral (14km/s) resolutions do not allow a proper analysis of the water maser emission, but suffice to investigate its association with the continuum sources. We have also studied the detection rate of HII regions in the two types of IRAS sources defined by Palla et (1991) on the basis of the IRAS colours: High and Low. No significant differences are found, with large detection rates (>90%) for both High and Low sources. We classify the millimeter and infrared sources in our fields in three evolutionary stages following the scheme presented by ...

Physical properties of high-mass clumps in different stages of evolution

Context. The details of the process of massive star formation are still elusive. A complete characterization of the first stages of the process from an observational point of view is needed to constrain theories on the subject. In the past 20 years we have made a thorough investigation of colour-selected IRAS sources over the whole sky. The sources in the northern hemisphere were studied in detail and used to derive an evolutionary sequence based on their spectral energy distribution. Aims. To investigate the first stages of the process of high-mass star formation, we selected a sample of massive clumps previously observed with the Swedish-ESO Submillimetre Telescope at 1.2 mm and with the ATNF Australia Telescope Compact Array at 1.3 cm. We want to characterize the physical conditions in such sources, and test whether their properties depend on the evolutionary stage of the clump. Methods. With ATCA we observed the selected sources in the NH3(1,1) and (2,2) transitions and in the H2O(616 − 523) maser line. Ammonia lines are a very good temperature probe that allow us to accurately determine the mass and the column, volume, and surface densities of the clumps. We also collected all data available to construct the spectral energy distribution of the individual clumps and to determine if star formation is already occurring through observations of its most common signposts, thus putting constraints on the evolutionary stage of the source. We fitted the spectral energy distribution between 1.2 mm and 70 μm with a modified black body to derive the dust temperature and independently determine the mass. Results. We find that the clumps are cold (T ∼ 10 − 30 K), massive (M ∼ 102 − 103 M ), and dense (n(H2) & 105 cm−3) and that they have high column densities (N(H2) ∼ 1023 cm−2). All clumps appear to be potentially able to form high-mass stars. The most massive clumps appear to be gravitationally unstable, if the only sources of support against collapse are turbulence and thermal pressure, which possibly indicates that the magnetic field is important in stabilizing them. Conclusions. After investigating how the average properties depend on the evolutionary phase of the source, we find that the temperature and central density progressively increase with time. Sources likely hosting a ZAMS star show a steeper radial dependence of the volume density and tend to be more compact than starless clumps.

Properties of dense cores in clustered massive star-forming regions at high angular resolution

We aim at characterising dense cores in the clustered environments associated with massive star-forming regions. For this, we present an uniform analysis of VLA NH3(1,1) and (2,2) observations towards a sample of 15 massive star-forming regions, where we identify a total of 73 cores, classify them as protostellar, quiescent starless, or perturbed starless, and derive some physical properties. The average sizes and ammonia column densities are 0.06 pc and 10^15 cm^-2, respectively, with no significant differences between the starless and protostellar cores, while the linewidth and rotational temperature of quiescent starless cores are smaller, 1.0 km/s and 16 K, than those of protostellar (1.8 km/s, 21 K), and perturbed starless (1.4 km/s, 19 K) cores. Such linewidths and temperatures for these quiescent starless cores in the surroundings of massive stars are still significantly larger than the typical values measured in starless cores of low-mass star-forming regions, implying an important non-thermal component. We confirm at high angular resolutions the correlations previously found with single-dish telescopes between the linewidth, the temperature of the cores, and the bolometric luminosity. In addition, we find a correlation between the temperature of each core and the incident flux from the most massive star in the cluster, suggesting that the large temperatures measured in the starless cores of our sample could be due to heating from the nearby massive star. A simple virial equilibrium analysis seems to suggest a scenario of a self-similar, self-graviting, turbulent, virialised hierarchy of structures from clumps (0.1-10 pc) to cores (0.05 pc). A closer inspection of the dynamical state taking into account external pressure effects, reveal that relatively strong magnetic field support may be needed to stabilise the cores, or that they are unstable and thus on the verge of collapse.

YOUNG STARLESS CORES EMBEDDED IN THE MAGNETICALLY DOMINATED PIPE NEBULA

The Pipe Nebula is a massive, nearby dark molecular cloud with a low star formation efficiency which makes it a good laboratory in which to study the very early stages of the star formation process. The Pipe Nebula is largely filamentary and appears to be threaded by a uniform magnetic field at scales of a few parsecs, perpendicular to its main axis. The field is only locally perturbed in a few regions, such as the only active cluster-forming core B59. The aim of this study is to investigate primordial conditions in low-mass pre-stellar cores and how they relate to the local magnetic field in the cloud. We used the IRAM 30 m telescope to carry out a continuum and molecular survey at 3 and 1 mm of early- and late-time molecules toward four selected starless cores inside the Pipe Nebula. We found that the dust continuum emission maps trace the densest regions better than previous Two Micron All Sky Survey (2MASS) extinction maps, while 2MASS extinction maps trace the diffuse gas better. The properties of the cores derived from dust emission show average radii of ~0.09 pc, densities of ~1.3×105 cm–3, and core masses of ~2.5 M ☉. Our results confirm that the Pipe Nebula starless cores studied are in a very early evolutionary stage and present a very young chemistry with different properties that allow us to propose an evolutionary sequence. All of the cores present early-time molecular emission with CS detections in the whole sample. Two of them, cores 40 and 109, present strong late-time molecular emission. There seems to be a correlation between the chemical evolutionary stage of the cores and the local magnetic properties that suggests that the evolution of the cores is ruled by a local competition between the magnetic energy and other mechanisms, such as turbulence.

The NH2D/NH3 ratio toward pre-protostellar cores around the UCH II region in IRAS 20293+3952

Context. The deuterium fractionation, D frac , has been proposed as an evolutionary indicator in pre-protostellar and protostellar cores of low-mass star-forming regions. Aims. We investigate D frac . with high angular resolution, in the cluster environment surrounding the UCH II region IRAS 20293+3952. Methods. We performed high angular resolution observations with the IRAM Plateau de Bure Interferometer (PdBI) of the ortho-NH 2 D 1 11 ―1 01 line at 85.926 GHz and compared them with previously reported VLA NH 3 data. Results. We detected strong NH 2 D emission toward the pre-protostellar cores identified in NH 3 and dust emission, all located in the vicinity of the UCH II region IRAS 20293+3952. We found high values of D frac ≃0.1―0.8 in all the pre-protostellar cores and low values, D frac < 0.1, associated with young stellar objects. Conclusions. The high values of D frac in pre-protostellar cores could be indicative of evolution, although outflow interactions and UV radiation could also play a role.

Three intermediate-mass young stellar objects with different properties emerging from the same natal cloud in IRAS 00117+6412

Aims. Our main aim is to study the influence of the initial conditions of a cloud in the intermediate/high-mass star formation process. Methods. We observed with the VLA, PdBI, and SMA the centimeter and millimeter continuum, N2H + (1–0), and CO (2–1) emission associated with a dusty cloud harboring a nascent cluster with intermediate-mass protostars. Results. At centimeter wavelengths we found a strong source, tracing a UCH ii region, at the eastern edge of the dusty cloud, with a shell-like structure, and with the near-infrared counterpart falling in the center of the shell. This is presumably the most massive source of the forming cluster. About 15 �� to the west of the UCH ii region and well embedded in the dusty cloud, we detected a strong millimeter source, MM1, associated with centimeter and near-infrared emission. MM1 seems to be driving a prominent high-velocity CO bipolar outflow elongated in the northeast-southwest direction, and is embedded in a ridge of dense gas traced by N2H + , elongated roughly in the same direction as the outflow. We estimated that MM1 is an intermediate-mass source in the Class 0/I phase. About 15 �� to the south of MM1, and still more deeply embedded in the dusty cloud, we detected a compact millimeter source, MM2, with neither centimeter nor near-infrared emission, but with water maser emission. MM2 is associated with a clump of N2H + , whose kinematics reveal a clear velocity gradient and additionally we found signposts of infall motions. MM2, being deeply embedded within the dusty cloud, with an associated water maser but no hints of CO outflow emission, is an intriguing object, presumably of intermediate mass. Conclusions. The UCH ii region is found at the border of a dusty cloud which is currently undergoing active star formation. Two intermediate-mass protostars in the dusty cloud seem to have formed after the UCH ii region and have different properties related to the outflow phenomenon. Thus, a single cloud with similar dust emission and similar dense gas column densities seems to be forming objects with different properties, suggesting that the initial conditions in the cloud are not determining all the star formation process.

N2H+ depletion in the massive protostellar cluster AFGL 5142

Aims. We aim at investigating the NH3/N2H + abundance ratio toward the high-mass star-forming region AFGL 5142 with high angular resolution in order to study whether the NH3/N2H + ratio behaves similarly to the low-mass case, for which the ratio decreases from starless cores to cores associated with young stellar objects (YSOs). Methods. CARMA was used to observe the 3.2 mm continuum and N2H + (1-0) emission toward AFGL 5142. We used NH3 (1, 1) and (2, 2), as well as HCO + (1-0) and H 13 CO + (1-0) data available from the literature, to study the chemical environment. Additionally, we performed a time-dependent chemical modeling of the region. Results. The 3.2 mm continuum emission reveals a dust condensation of ∼23 Massociated with the massive YSOs, deeply embedded in the strongest NH3 core (hereafter central core). The dense gas emission traced by N2H + reveals two main cores, the western core of ∼0.08 pc in size and the eastern core of ∼0.09 pc, surrounded by a more extended and complex structure of ∼0.5 pc, mimicking the morphology of the NH3 emission. The two cores are located to the west and to the east of the 3.2 mm dust condensation. Toward the central core the N2H + emission drops significantly, indicating a clear chemical differentiation in the region. The N2H + column density in the central core is one order of magnitude lower than in the western and eastern cores. Furthermore, we found low values of the NH3/N2H + abundance ratio ∼50-100 toward the western and eastern cores and high values up to 1000 associated with the central core. The chemical model used to explain the differences seen in the NH3/N2H + ratio indicates that density along with temperature is a key parameter in determining the abundances of both NH3 and N2H + . The high density (n � 10 6 cm −3 ) and temperature (T � 70 K) reached in the central core allow molecules such as CO to evaporate from grain mantles. The CO desorption causes a significant destruction of N2H + , which favors the formation of HCO + . This result is supported by our observations, which show that N2H + and HCO + are anticorrelated in the central core. The observed values of the NH3/N2H + ratio in the central core can be reproduced by our model for times of t � 4.5−5.3 × 10 5 yr, while in the western and eastern cores the NH3/N2H + ratio can be reproduced by our model for times in the range 10 4 −3 × 10 6 yr. Conclusions. The NH3/N2H + abundance ratio in AFGL 5142 does not follow the same trend as in regions of low-mass star formation mainly because of the high temperature reached in hot cores.

A necklace of dense cores in the high-mass star forming region G35.20-0.74 N: ALMA observations

Context. The formation process of high-mass stars (with masses >8 M_⊙) is still poorly understood, and represents a challenge from both the theoretical and observational points of view. The advent of the Atacama Large Millimeter Array (ALMA) is expected to provide observational evidence to better constrain the theoretical scenarios. Aims. The present study aims at characterizing the high-mass star forming region G35.20−0.74 N, which is found associated with at least one massive outflow and contains multiple dense cores, one of them recently found associated with a Keplerian rotating disk. Methods. We used the radio-interferometer ALMA to observe the G35.20−0.74 N region in the submillimeter continuum and line emission at 350 GHz. The observed frequency range covers tracers of dense gas (e.g., H^(13)CO^+, C^(17)O), molecular outflows (e.g., SiO), and hot cores (e.g., CH_3CN, CH_3OH). These observations were complemented with infrared and centimeter data. Results. The ALMA 870 μm continuum emission map reveals an elongated dust structure (~0.15 pc long and ~0.013 pc wide; full width at half maximum) perpendicular to the large-scale molecular outflow detected in the region, and fragmented into a number of cores with masses ~1–10 M_⊙ and sizes ~1600 AU (spatial resolution ~960 AU). The cores appear regularly spaced with a separation of ~0.023 pc. The emission of dense gas tracers such as H^(13)CO^+ or C^(17)O is extended and coincident with the dust elongated structure. The three strongest dust cores show emission of complex organic molecules characteristic of hot cores, with temperatures around 200 K, and relative abundances 0.2–2 × 10^(-8) for CH_3CN and 0.6–5 × 10^(-6) for CH_3OH. The two cores with highest mass (cores A and B) show coherent velocity fields, with gradients almost aligned with the dust elongated structure. Those velocity gradients are consistent with Keplerian disks rotating about central masses of 4–18 M_⊙. Perpendicular to the velocity gradients we have identified a large-scale precessing jet/outflow associated with core B, and hints of an east-west jet/outflow associated with core A. Conclusions. The elongated dust structure in G35.20−0.74 N is fragmented into a number of dense cores that may form high-mass stars. Based on the velocity field of the dense gas, the orientation of the magnetic field, and the regularly spaced fragmentation, we interpret this elongated structure as the densest part of a 1D filament fragmenting and forming high-mass stars.