LOW-VELOCITY SHOCKS TRACED BY EXTENDED SiO EMISSION ALONG THE W43 RIDGES: WITNESSING THE FORMATION OF YOUNG MASSIVE CLUSTERS (original) (raw)
Related papers
SiO in G34.26: Outflows and shocks in a high mass star forming region
Astronomy and Astrophysics, 2001
We have looked for SiO emission as evidence of shocks in the high mass star formation region G34.26+0.15. JCMT, VLA and FCRAO observations show that SiO emission is widespread across the region. The SiO emission highlights a massive, collimated outflow and other regions where stellar winds are interacting with molecular clumps. As in other star forming regions, there is also SiO at ambient velocities which is related to the outflow activity. No strong SiO abundance enhancement was measured in either the outflow or the low velocity gas, though abundances up to 10 −8 are possible if the SiO is locally enhanced in clumps and optically thick. SiO emission is not detected from the hot core itself, indicating either that SiO is not strongly enhanced in the hot core or that column densities in the region where grain mantle evaporation has taken place are low. In line of sight spiral arm clouds, we measure a SiO abundance of 0.4-2 × 10 −10 , consistent with previous estimates for quiescent clouds.
SO and SiO emission around the young cluster in the CB 34 globule
Monthly Notices of the Royal Astronomical Society, 2002
The globule CB 34, which harbours a cluster of class 0 young stellar object (YSO) protostars, has been investigated through a multiline SO and SiO survey at millimetre wavelengths. The SO data reveal that the globule consists of three quiescent high-density (,10 5 cm 23) clumps, labelled A, B and C, with sizes of ,0:2-0:3 pc. The SiO data provide evidence for highvelocity gas across the globule. Most likely, the high-velocity gas is distributed in three distinct high-velocity outflows associated with the YSOs in each of the three clumps. Highvelocity SO features have been detected only towards the two brightest SiO outflows. These broad SO components exhibit spatial and spectral distributions which are consistent with those of the SiO emission, so they can also be used as tracers of the outflows. The comparison between the spatial and spectral properties of the SO and SiO emissions in the three clumps suggests different evolutionary stages for the embedded YSOs. In particular, the YSO associated with clump C exhibits some peculiarities, namely smaller SiO linewidths, lower SiO column densities, a lack of extended SiO structure and of SO wings, and the presence of a SO spatial distribution which is displaced with respect to the location of the YSO. This behaviour is well explained if the SiO and SO molecules which were produced at high velocities in the shocked region have been destroyed or slowed down because of the interaction with the ambient medium, and the chemistry is dominated again by lowtemperature reactions. Thus our observations strongly suggest that the YSO in clump C is in a more evolved phase than the other members of the cluster.
Star formation in the Vela molecular ridge
Astronomy and Astrophysics, 2007
Context. The Vela Molecular Ridge is one of the nearest intermediate-mass star forming regions, located within the galactic plane and outside the solar circle. Cloud D, in particular, hosts a number of small embedded young clusters. Aims. We present the results of a large-scale map in the dust continuum at 1.2 mm of a ∼ 1 • × 1 • area within cloud D. The main aim of the observations was to obtain a complete census of cluster-forming cores and isolated (both high-and low-mass) young stellar objects in early evolutionary phases. Methods. The bolometer array SIMBA at SEST was used to map the dust emission in the region with a typical sensitivity of ∼ 20 mJy/beam. This allows a mass sensitivity of ∼ 0.2 M ⊙ . The resolution is 24 ′′ , corresponding to ∼ 0.08 pc, roughly the radius of a typical young embedded cluster in the region. The continuum map is also compared to a large scale map of CO(1-0) integrated emission. Results. Using the CLUMPFIND algorithm, a robust sample of 29 cores has been obtained, spanning the size range 0.03 − 0.25 pc and the mass range 0.4 − 88 M ⊙ . The most massive cores are associated both with red IRAS sources and with embedded young clusters, and coincide with CO(1-0) integrated emission peaks. The cores are distributed according to a mass spectrum ∼ M −α and a mass-versus-size relation ∼ D x , with α ∼ 1.45 − 1.9 and x ∼ 1.1 − 1.7. They appear to originate in the fragmentation of gas filaments seen in CO(1-0) emission and their formation is probably induced by expanding shells of gas. The core mass spectrum is flatter than the Initial Mass Function of the associated clusters in the same mass range, suggesting further fragmentation within the most massive cores. A threshold A V ∼ 12 mag seems to be required for the onset of star formation in the gas.
A tale of two cores: triggered massive star formation in the bright-rimmed cloud SFO 75
Arxiv preprint astro-ph/ …, 2007
Context. Bright-rimmed clouds (BRCs) are isolated molecular clouds located on the edges of evolved HII regions. Star formation within the BRCs may have been triggered through the propagation of photoionisation-induced shocks driven by the expansion of the HII region. Aims. The main focus of this paper is to investigate the current level of star formation within one of these clouds and evaluate to what extent, if any, star formation may have been triggered. Methods. We present a detailed multi-wavelength study of the BRC SFO 75, including 1.3 cm and 1.2 mm continuum, and 13 CO and ammonia spectral line observations. To build up a comprehensive picture of the local environment we complement our observations with archival data from the 2MASS, GLIMPSE and IRAS surveys. Results. The 13 CO and 1.2 mm emission reveals the presence of a dense core located behind the bright rim of the cloud which is approximately coincident with that of the IRAS point source. From an analysis of the IRAS and 1.2 mm fluxes we derive a dust temperature of ∼ 30 K, a luminosity of L bol = 1.6 × 10 4 L ⊙ and estimate the core mass to be ∼ 570 M ⊙ . The higher resolution ammonia observations resolve the 1.2 mm core into two distinct cores, one directly behind the cloud's rim (Core A) and the second located slightly farther back (Core B). These have masses of 8-15 M ⊙ and 3.5-7 M ⊙ for Core A and Core B respectively, which are significantly larger then their virial masses. Comparing the morphology of Core A with that of the photon-dominated region and ionised boundary layer leaves little doubt that it is being strongly affected by the ionisation front. 2MASS and GLIMPSE archive data which reveal a small cluster of three deeply embedded (A v ∼ 20 mag) high-and intermediate-mass young stellar objects towards Core A leads us to conclude that the star formation found towards this core has been triggered. In stark contrast, Core B appears to have a much simpler, almost spherical, morphology. No stars are found towards Core B. We find evidence supporting the presence of shocked gas within the surface layers of the cloud which appears to extend to midway between the two ammonia cores.
The Formation of High-Mass Stars: from High-Mass Clumps to Accretion Discs and Molecular Outflows
High-mass stars play a significant role in the evolution of the Universe and the process that leads to the formation of such objects is still an open question in Astrophysics. The details of the structures connected to the central sources, such as the circumstellar discs and the morphology of the jets at their launching points, still lack of observational evidence. In this thesis, the high-mass star forming process is investigated in terms of the evolution of high-mass clumps selected from the ATLASGAL survey based on their 12 CO emission in the sub-millimetre. While single-dish sub-millimetre observations provide a large-scale view of the high-mass star formation process, higher angular resolution observations are required to disentangle the details of the protostars within the clumps. For this, threedimensional infrared spectroscopy was obtained for a group of RMS sources to characterise the circumstellar environment of high-mass YSOs in linear scales of ∼100-1000 AU. The ATLASGAL TOP100 sample offers a unique opportunity to analyse a statistically complete sample of high-mass clumps at different evolutionary stages. APEX data of three rotational J transitions of the CO (the CO (4-3), CO (6-5) and CO (7-6)) were used to characterise the properties of their warm gas (155 K) content and to derive the relations between the CO and the clump properties. The CO line luminosities were derived and the analysis indicated that the CO emission increases as a function of the evolutionary stage of the clumps (from infrared-weak to H ii regions) and as a function of the bolometric luminosity (L bol) and mass of the sources (M clump). The comparison of the TOP100 with low-mass objects observed in the CO (6-5) and CO (7-6), together with CO (10-9) data observed for a complementary sample of objects indicated that the dependency of the CO luminosity (L CO) with the bolometric luminosity of the sources gets steeper towards higher-J transitions. Although the CO luminosity of more luminous clumps are systematically larger than the values obtained for the less luminous sources, the individual analysis of each subsample suggests a similar dependency of L CO versus L bol for each luminosity regime. Finally, the presence of high-velocity CO emission observed for the TOP100 suggests that ∼85% of the sources are driving molecular outflows. The selection of isolated high-mass objects undergoing mass accretion is fundamental to investigate if these objects are formed through an accretion disc or if they are formed by merging of low-mass YSOs. The near-infrared window provides one of the best opportunities to investigate the interior of the sub-mm clumps and study in details their individual members. Thanks to the relatively high-resolution obtained in the K-band and the moderate reddening effectsin the K-band, a sample of eight (8) HMYSOs exhibiting large-scale H 2 outflows were selected to follow-up K-band spectroscopic observations using the NIFS spectrometer (Gemini North). All sources exhibit extended continuum emission and exhibit atomic and molecular transitions typical of embedded objects, such as Brγ, H 2 and the CO lines. The H 2 lines are tracing the launching point of the large-scale jets in scales of ∼ 100 AU in five of eight sources (63%). The identification of jets at such small scales indicates that these objects are still undergoing mass accretion. The Brγ emission probes the ionised gas around the HMYSOs. The analysis of the Brγ spectro-astrometry at sub-pixel scales suggests that the line arises from the cavity of the outflows or from rotating structures perpendicular to the H 2 jets (i.e., disc). Five sources also exhibit CO emission features (63%), and three HMYSOs display CO absorption features (38%), indicating that they are likely associated with circumstellar discs. By further investigating the kinematics of the spatially resolved CO absorption features, the Keplerian mass of three sources was estimated in 5±3, 8±5 and 30±10 M ⊙. These results support that high-mass stars are formed through discs, similarly as observed towards low-mass stars. The comparison between the collimation degree of the molecular jets or outflows detected in the NIFS data with their large-scale counterparts indicate that these structures present a relatively wide range of collimation degrees.
Multiple Outflows in the High-mass Cluster-forming Region G25.82–0.17
The Astrophysical Journal, 2020
We present results of continuum and spectral line observations with ALMA and 22 GHz water (H 2 O) maser observations using KaVA and VERA toward a high-mass star-forming region, G25.82-0.17. Multiple 1.3 mm continuum sources are revealed, indicating the presence of young stellar objects (YSOs) at different evolutionary stages, namely an ultra-compact Hii region, G25.82-E, a high-mass young stellar object (HM-YSO), G25.82-W1, and starless cores, G25.82-W2 and G25.82-W3. Two SiO outflows, at N-S and SE-NW orientations, are identified. The CH 3 OH 8 −1-7 0 E line, known to be a class I CH 3 OH maser at 229 GHz is also detected showing a mixture of thermal and maser emission. Moreover, the H 2 O masers are distributed in a region ∼ 0.25 ′′ shifted from G25.82-W1. The CH 3 OH 22 4-21 5 E line shows a compact ring-like structure at the position of G25.82-W1 with a velocity gradient, indicating a rotating disk or envelope. Assuming Keplerian rotation, the dynamical mass of G25.82-W1 is estimated to be >25 M ⊙ and the total mass of 20 M ⊙-84 M ⊙ is derived from the 1.3 mm continuum emission. The driving source of the N-S SiO outflow is G25.82-W1 while that of the SE-NW SiO outflow is uncertain. Detection of multiple high-mass starless/protostellar cores and candidates without low-mass cores implies that HM-YSOs could form in individual high-mass cores as predicted by the turbulent core accretion model. If this is the case, the high-mass star formation process in G25.82 would be consistent with a scaled-up version of low-mass star formation.
The Astrochemical Evolution of Giant molecular Clouds: Shock Dynamics and Gas-Grain Interactions
2010
"The cold dense phase of the interstellar medium consists of giant molecular clouds (GMCs), with masses from several hundred thousand to several million times the mass of the sun and gas kinetic temperatures of 10 K. They consist primarily of gaseous molecular hydrogen H2, along with numerous trace molecules - ranging from simple molecules such as CO, CN, NH3, and CH4 up to more complex organic molecules such as methanol (CH3OH) and amino acetonitrile (NH2CH2CN). Interstellar dust, composed of graphite, carbon and/or silicate compounds, is also an important constituent in the interstellar medium. Approximately 1 % of the mass of the interstellar medium is in the form of dust and the remainder is in gas. These dust grains, with dimensions of a few tenths of microns, play a vital role in influencing the physical and chemical state of the interstellar medium -- from the thermodynamics and chemistry of the gas, to the dynamics of star formation. For instance, the heating and cooling mechanisms in GMCs are based on the collisional heating and cooling and radiative cooling and heating which takes place in the coupled gas-grain system. Moreover, the surfaces of dust grain catalyze chemical reaction pathways leading to the formation of both simple (eg. H2) and complex (eg. CH3OH) molecules. Models which include only thermal desorption predict that molecules in the gaseous phase of the GMCs should rapidly freeze out, or desorb, onto the surfaces of dust grains on time scales much shorter than the lifetime of the GMC. At the mean temperatures of the GMCs, however, we need to include non-thermal desorption mechanisms because the thermal energy is significantly less than the surface molecular binding energy and hence the thermal desorption rate is negligible. We calculate the number of far ultraviolet (FUV) photons produced from cosmic-ray protons in ionization equilibrium by the Prasad-Tarafdar effect, in which neutral H2 is collisionally excited by high-energy cosmic-ray-ionized electrons, and by recombination. Using this background FUV field as a boundary condition, we, for the first time, consider the effect of radiative precursors from interstellar shocks. Specifically, we demonstrate that the post-shock compression enhances the production of FUV photons in a post-shock region where the dust grains slip relative to the gas. Inclusion of this process in our models results in the enhancement of photodesorption of molecules from grain surfaces. Finally, we estimate the total gas-phase abundances in GMCs including the combined effect of adsorption and the newly-discovered effect of shock-enhanced FUV photodesorption. "
SiO Outflows as Tracers of Massive Star Formation in Infrared Dark Clouds
The Astrophysical Journal, 2021
To study the early phases of massive star formation, we present ALMA observations of SiO(5-4) emission and VLA observations of 6 cm continuum emission towards 32 Infrared Dark Cloud (IRDC) clumps, which are spatially resolved down to 0.05 pc. Out of the 32 clumps observed, we have detected SiO emission in 20 clumps, and in 11 of them it is relatively strong and likely tracing protostellar outflows. Some SiO outflows are collimated, while others are less well ordered. There is evidence for episodic ejection events, as well as multiple outflows originating from scales of 0.1 pc. For the six strongest SiO outflows, we estimate basic outflow properties. We do not see clear dependence of the degree of collimation of the outflows on core mass, luminosity and evolutionary stage. In our entire sample, where there is SiO emission, we always find 1.3 mm continuum emission and some infrared emission nearby, but not vice versa. We build the spectral energy distributions (SEDs) of all the cores with 1.3 mm continuum emission and fit them with radiative transfer (RT) models. The low luminosities and stellar masses returned by SED fitting suggest these are early stage protostars. We see a slight trend of increasing SiO line luminosity with bolometric luminosity, which suggests more powerful shocks in the vicinity of more massive YSOs. However, we do not see a clear relation between the SiO luminosity and the evolutionary stage indicated by L/M. We conclude that as a protostar approaches a bolometric luminosity of ∼ 10 2 L , the shocks in the outflow are generally strong enough to form SiO emission. The VLA 6 cm observations toward the 15 clumps with the strongest SiO emission detect emission in four clumps, which is likely to be shock ionized jets associated with the more massive of these protostellar cores.
High-mass star formation within the bright-rimmed cloud SFO 79
Arxiv preprint astro-ph/ …, 2004
We report Radio Recombination Line (RRL) and continuum observations toward the IRAS point source 16362-4845, embedded within the Bright-Rimmed Cloud (BRC) SFO 79, a small molecular cloud lying at the edge of the HII region RCW 108. High resolution observations of the H92α hydrogen recombination line and of the continuum emission (3.6 and 6 cm) confirm the presence of a resolved Ultra Compact (UC) HII region embedded within the molecular cloud. The integrated radio fluxes suggest the source of the ionisation to be an O9 Zero Age Main Sequence (ZAMS) star. Millimetre observations of 12 CO, 13 CO and C 18 O (J =1-0) molecular lines reveal the presence of a molecular condensation offset ∼30 ′′ to the north of the IRAS position on the boundary of the UC HII region. Analysis of 2MASS data has led to the identification of a small IR cluster of Young Stellar Objects (YSOs) that are positionally coincident with the UC HII region, lying to the south east of the peak of the radio emission. Moreover, the UC HII region appears to be extended in the direction of the IR cluster, which suggests that the radio emission and the IR cluster are in some way related to each other. MSX 8.3 µm and 21.3 µm images have been used to trace the large scale structure of the BRC, revealing the presence of a Photo Dominated Region (PDR) and three embedded thermal sources within the molecular cloud. The PDR has a plane parallel morphology which correlates extremely well with the morphology of the ionised gas traced by the optical emission. The three thermal sources (labelled A, B, C) all lie at a similar projected distance from the interface between the HII region and the molecular gas of the cloud. Thermal sources A and C are positionally coincident with the IRAS point sources 16362-4845 and 16362-4841 respectively, both of which have IRAS colours consistent with the presence of UC HII regions. Given that UC HII regions are relatively short lived (∼10 5 yrs) it is reasonable to suggest that these two UC HII regions are of a similar age. The alignment of the three thermal sources along a line parallel to the bright rim suggests that they could have been triggered by the propagation of a plane parallel shock through the cloud.
Astronomy & Astrophysics, 2011
The filament IRDC G035.39-00.33 in the W48 molecular complex is one of the darkest infrared clouds observed by Spitzer. It has been observed by the PACS (70 and 160 µm) and SPIRE (250, 350, and 500 µm) cameras of the Herschel Space Observatory as part of the W48 molecular cloud complex in the framework of the HOBYS key programme. The observations reveal a sample of 28 compact sources (deconvolved FWHM sizes <0.3 pc) complete down to ∼5 M ⊙ in G035.39-00.33 and its surroundings. Among them, 13 compact sources are massive dense cores with masses >20 M ⊙ . The cloud characteristics we derive from the analysis of their spectral energy distributions are masses of 20 − 50 M ⊙ , sizes of 0.1-0.2 pc, and average densities of 2 − 20 × 10 5 cm −3 , which make these massive dense cores excellent candidates to form intermediate-to high-mass stars. Most of the massive dense cores are located inside the G035.39-00.33 ridge and host IR-quiet high-mass protostars. The large number of protostars found in this filament suggests that we are witnessing a mini-burst of star formation with an efficiency of ∼15% and a rate density of ∼40 M ⊙ yr −1 kpc −2 within ∼8 pc 2 , a large area covering the full ridge. Part of the extended SiO emission observed towards G035.39-00.33 is not associated with obvious protostars and may originate from low-velocity shocks within converging flows, as advocated by previous studies.