G11.92-0.61-MM2 : a bonafide massive prestellar core?

Supported by NSF AAPF (C.J.C., AST-1003134) and ERC (A.V., PALs 320620).Core accretion models of massive star formation require the existence of stable massive starless cores, but robust observational examples of such objects have proven elusive. We report subarcsecond-resolution Submillimeter Array (SMA) 1.3 mm, 1.1 mm, and 0.88 mm and Very Large Array 1.3 cm observations of an excellent massive starless core candidate, G11.92–0.61-MM2, initially identified in the course of studies of GLIMPSE Extended Green Objects (EGOs). Separated by ~7 farcs 2 from the nearby MM1 protostellar hot core, MM2 is a strong, compact dust continuum source (submillimeter spectral index α = 2.6 ± 0.1), but is devoid of star formation indicators. In contrast to MM1, MM2 has no masers, no centimeter continuum, and no (sub)millimeter wavelength line emission in ~24 GHz of bandwidth observed with the SMA, including N2H+(3-2), HCO+(3-2), and HCN(3-2). Additionally, there is no evidence for an outflow driven by MM2. The (sub)millimeter spectral energy distribution of MM2 is best fit with a dust temperature of ~17-19 K and luminosity of ~5-7 L☉. The combined physical properties of MM2, as inferred from its dust continuum emission, are extreme: M ≳ 30 M☉ within a radius 1025 cm–2 and nH_2 >109 cm–3. Comparison of the molecular abundance limits derived from our SMA observations with gas-grain chemical models indicates that extremely dense (n(H) ≫ 108 cm–3), cold (<20 K) conditions are required to explain the lack of observed (sub)millimeter line emission, consistent with the dust continuum results. Our data suggest that G11.92–0.61-MM2 is the best candidate for a bonafide massive prestellar core found to date, and a promising target for future higher-sensitivity observations.Publisher PDFPeer reviewe

The extraordinary outburst in the massive protostellar system NGC 6334 I-MM1 : strong increase in mid-infrared continuum emission

Financial support for this work was provided by NASA through award #07_0156 issued by USRA. Based in part on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO program 089.C-0852(A).In recent years, dramatic outbursts have been identified toward massive protostars via infrared and millimeter dust continuum and molecular maser emission. The longest lived outburst (>6 yr) persists in NGC 6334 I-MM1, a deeply embedded object with no near-IR counterpart. Using FORCAST and HAWC+ on SOFIA, we have obtained the first mid-IR images of this field since the outburst began. Despite being undetected in pre-outburst ground-based 18 μm images, MM1 is now the brightest region at all three wavelengths (25, 37, and 53 μm), exceeding the UCHII region MM3 (NGC 6334 F). Combining the SOFIA data with ALMA imaging at four wavelengths, we construct a spectral energy distribution of the combination of MM1 and the nearby hot core MM2. The best-fit Robitaille radiative transfer model yields a luminosity of (4.9 ± 0.8) × 104 L⊙. Accounting for an estimated pre-outburst luminosity ratio MM1:MM2 = 2.1 ± 0.4, the luminosity of MM1 has increased by a factor of 16.3 ± 4.4. The pre-outburst luminosity implies a protostar of mass 6.7 M⊙, which can produce the ionizing photon rate required to power the pre-outburst HCHII region surrounding the likely outbursting protostar MM1B. The total energy and duration of the outburst exceed the S255IR-NIRS3 outburst by a factor of 3, suggesting a different scale of event involving expansion of the protostellar photosphere (to 20 R⊙), thereby supporting a higher accretion rate (0.0023 M⊙ yr−1) and reducing the ionizing photon rate. In the grid of hydrodynamic models of Meyer et al., the combination of outburst luminosity and magnitude (3) places the NGC 6334 I-MM1 event in the region of moderate total accretion (~0.1–0.3 M⊙) and hence long duration (~40–130 yr).PostprintPeer reviewe

ALMA observations of the Extended Green Object G19.01-0.03 - I. A Keplerian disc in a massive protostellar system

Interstellar matter and star formatio

RGSM Skoplje 2015.

We present 1.05 mm ALMA observations of the deeply embedded high-mass protocluster G11.92-0.61, designed to search for low-mass cores within the accretion reservoir of the massive protostars. Our ALMA mosaic, which covers an extent of ~0.7 pc at sub-arcsecond (~1400 au) resolution, reveals a rich population of 16 new millimetre continuum sources surrounding the three previously-known millimetre cores. Most of the new sources are located in the outer reaches of the accretion reservoir: the median projected separation from the central, massive (proto)star MM1 is ~0.17 pc. The derived physical properties of the new millimetre continuum sources are consistent with those of low-mass prestellar and protostellar cores in nearby star-forming regions: the median mass, radius, and density of the new sources are 1.3 Msun, 1600 au, and n(H2)~10^7 cm^-3. At least three of the low-mass cores in G11.92-0.61 drive molecular outflows, traced by high-velocity 12CO(3-2) (observed with the SMA) and/or by H2CO and CH3OH emission (observed with ALMA). This finding, combined with the known outflow/accretion activity of MM1, indicates that high- and low-mass stars are forming (accreting) simultaneously within this protocluster. Our ALMA results are consistent with the predictions of competitive-accretion-type models in which high-mass stars form along with their surrounding clusters.Comment: 17 pages, 5 figures, 5 tables, accepted for publication in MNRAS; v2: proper acknowledgement to NRAO adde

Physical Processes in Star Formation

© 2020 Springer-Verlag. The final publication is available at Springer via https://doi.org/10.1007/s11214-020-00693-8.Star formation is a complex multi-scale phenomenon that is of significant importance for astrophysics in general. Stars and star formation are key pillars in observational astronomy from local star forming regions in the Milky Way up to high-redshift galaxies. From a theoretical perspective, star formation and feedback processes (radiation, winds, and supernovae) play a pivotal role in advancing our understanding of the physical processes at work, both individually and of their interactions. In this review we will give an overview of the main processes that are important for the understanding of star formation. We start with an observationally motivated view on star formation from a global perspective and outline the general paradigm of the life-cycle of molecular clouds, in which star formation is the key process to close the cycle. After that we focus on the thermal and chemical aspects in star forming regions, discuss turbulence and magnetic fields as well as gravitational forces. Finally, we review the most important stellar feedback mechanisms.Peer reviewedFinal Accepted Versio

Subarcsecond imaging of the NGC 6334 I(n) protocluster:two dozen compact sources and a massive disk candidate

Using the Submillimeter Array (SMA) and Karl G. Jansky Very Large Array, we have imaged the massive protocluster NGC 6334 I(N) at high angular resolution (0″5 ~ 650 AU) from 6 cm to 0.87 mm, detecting 18 new compact continuum sources. Three of the new sources are coincident with previously identified H2O masers. Together with the previously known sources, these data bring the number of likely protocluster members to 25 for a protostellar density of ~700 pc–3. Our preliminary measurement of the Q-parameter of the minimum spanning tree is 0.82—close to the value for a uniform volume distribution. All of the (nine) sources with detections at multiple frequencies have spectral energy distributions consistent with dust emission, and two (SMA 1b and SMA 4) also have long wavelength emission consistent with a central hypercompact H II region. Thermal spectral line emission, including CH3CN, is detected in six sources: LTE model fitting of CH3CN (J = 12-11) yields temperatures of 72-373 K, confirming the presence of multiple hot cores. The fitted LSR velocities range from –3.3 to –7.0 km s–1, with an unbiased mean square deviation of 2.05 km s–1, implying a protocluster dynamical mass of 410 ± 260 M☉. From analysis of a wide range of hot core molecules, the kinematics of SMA 1b are consistent with a rotating, infalling Keplerian disk of diameter 800 AU and enclosed mass of 10-30 M☉ that is perpendicular (within 1°) to the large-scale bipolar outflow axis. A companion to SMA 1b at a projected separation of 0″45 (590 AU; SMA 1d), which shows no evidence of spectral line emission, is also confirmed. Finally, we detect one 218.4400 GHz and several 229.7588 GHz Class-I CH3OH masers

VLA and ALMA Imaging of the Massive Prestellar Core G11.92-0.61 MM2

We have obtained new Jansky Very Large Array (VLA) observations at X, K, and Ka bands (3 cm, 1.3 cm, and 0.9 cm) which have resolved the continuum emission from the most promising candidate for a massive pre-stellar core discovered to date: G11.92–0.61 MM2. As described in Cyganowski et al. ([1]), this bright dust continuum source (190 mJy at 1.1 mm) exhibits no spectral line emission in sub-arcsecond-resolution Submillimeter Array (SMA) images across 24 GHz of bandwidth, including the typical tracers CO, HCN, HCO+, and N2H+. Astrochemical models require high density (> 109 cm−3) and low temperature (< 20 K) to explain the rare chemistry of this massive (M ≥ 30 M⊙) object, which may exist in a fleeting evolutionary state. This source is well detected and elongated in VLA Ka-band (9 mm) continuum image with a 0.25′′ beam (800 AU), is marginally detected in poorer resolution (1) K-band (1.3 cm) data, and is undetected at X-band (3 cm) with 0.25′′ resolution. In combination with existing SMA millimeter wavelength data, our results provide an accurate spectral energy distribution of this source, constraining the dust grain emissivity index to 1.0–1.6 and the luminosity to 3–37 L⊙. Preliminary results from ALMA Band 7 images confirm that the dust emission from MM2 is resolved in an east-west direction

G11.92-0.61-MM2:a bonafide massive prestellar core?

Core accretion models of massive star formation require the existence of stable massive starless cores, but robust observational examples of such objects have proven elusive. We report subarcsecond-resolution Submillimeter Array (SMA) 1.3 mm, 1.1 mm, and 0.88 mm and Very Large Array 1.3 cm observations of an excellent massive starless core candidate, G11.92–0.61-MM2, initially identified in the course of studies of GLIMPSE Extended Green Objects (EGOs). Separated by ~7 farcs 2 from the nearby MM1 protostellar hot core, MM2 is a strong, compact dust continuum source (submillimeter spectral index α = 2.6 ± 0.1), but is devoid of star formation indicators. In contrast to MM1, MM2 has no masers, no centimeter continuum, and no (sub)millimeter wavelength line emission in ~24 GHz of bandwidth observed with the SMA, including N2H+(3-2), HCO+(3-2), and HCN(3-2). Additionally, there is no evidence for an outflow driven by MM2. The (sub)millimeter spectral energy distribution of MM2 is best fit with a dust temperature of ~17-19 K and luminosity of ~5-7 L☉. The combined physical properties of MM2, as inferred from its dust continuum emission, are extreme: M ≳ 30 M☉ within a radius &lt;1000 AU, NH_2 &gt;1025 cm–2 and nH_2 &gt;109 cm–3. Comparison of the molecular abundance limits derived from our SMA observations with gas-grain chemical models indicates that extremely dense (n(H) ≫ 108 cm–3), cold (&lt;20 K) conditions are required to explain the lack of observed (sub)millimeter line emission, consistent with the dust continuum results. Our data suggest that G11.92–0.61-MM2 is the best candidate for a bonafide massive prestellar core found to date, and a promising target for future higher-sensitivity observations