A Mid-Infrared Census of Star Formation Activity in Bolocam Galactic Plane Survey Sources

We present the results of a search for mid-infrared signs of star formation activity in the 1.1 mm sources in the Bolocam Galactic Plane Survey (BGPS). We have correlated the BGPS catalog with available mid-IR Galactic plane catalogs based on the Spitzer Space Telescope GLIMPSE legacy survey and the Midcourse Space Experiment (MSX) Galactic plane survey. We find that 44% (3,712 of 8,358) of the BGPS sources contain at least one mid-IR source, including 2,457 of 5,067 (49%) within the area where all surveys overlap (10 deg < l < 65 deg). Accounting for chance alignments between the BGPS and mid-IR sources, we conservatively estimate that 20% of the BPGS sources within the area where all surveys overlap show signs of active star formation. We separate the BGPS sources into four groups based on their probability of star formation activity. Extended Green Objects (EGOs) and Red MSX Sources (RMS) make up the highest probability group, while the lowest probability group is comprised of "starless" BGPS sources which were not matched to any mid-IR sources. The mean 1.1 mm flux of each group increases with increasing probability of active star formation. We also find that the "starless" BGPS sources are the most compact, while the sources with the highest probability of star formation activity are on average more extended with large skirts of emission. A subsample of 280 BGPS sources with known distances demonstrates that mass and mean H_2 column density also increase with probability of star formation activity.Comment: 20 pages, 12 figures, 3 tables. Accepted for publication in ApJ. Full Table 2 will be available online through Ap

Understanding Accretion Outbursts in Massive Protostars through Maser Imaging

The bright maser emission produced by several molecular species at centimeter to long millimeter wavelengths provides an essential tool for understanding the process of massive star formation. Unimpeded by the high dust optical depths that affect shorter wavelength observations, the high brightness temperature of these emission lines offers a way to resolve accretion and outflow motions down to scales below $\sim$1 au in deeply embedded Galactic star-forming regions at kiloparsec distances. The recent identification of extraordinary accretion outbursts in two high-mass protostars, both of which were heralded by maser flares, has rapidly impacted the traditional view of massive protostellar evolution, leading to new hydrodynamic simulations that can produce such episodic outbursts. In order to understand how these massive protostars evolve in response to such events, larger, more sensitive ground-based centimeter wavelength interferometers are needed that can simultaneously image multiple maser species in the molecular gas along with faint continuum from the central ionized gas. Fiducial observations of a large sample of massive protostars will be essential in order to pinpoint the progenitors of future accretion outbursts, and to quantify the outburst-induced changes in their protostellar photospheres and outflow and accretion structures. Knowledge gained from these studies will have broader impact on the general topic of accretion onto massive objects.Comment: Science white paper submitted to the Astro2020 Decadal Survey. arXiv admin note: substantial text overlap with arXiv:1806.0698

ALMA observations of the Extended Green Object G19.01−-0.03: II. A massive protostar with typical chemical abundances surrounded by four low-mass prestellar core candidates

We present a study of the physical and chemical properties of the Extended Green Object (EGO) G19.01$-$0.03 using sub-arcsecond angular resolution Atacama Large Millimeter/submillimeter Array (ALMA) 1.05mm and Karl G. Jansky Very Large Array (VLA) 1.21cm data. G19.01$-$0.03 MM1, the millimetre source associated with the central massive young stellar object (MYSO), appeared isolated and potentially chemically young in previous Submillimeter Array observations. In our $\sim0.4''$-resolution ALMA data, MM1 has four low-mass millimetre companions within 0.12pc, all lacking maser or outflow emission, indicating they may be prestellar cores. With a rich ALMA spectrum full of complex organic molecules, MM1 does not appear chemically young, but has molecular abundances typical of high-mass hot cores in the literature. At the 1.05mm continuum peak of MM1, $\mathrm{N}(\mathrm{CH}_{3}\mathrm{OH})=(2.22\pm0.01)\times10^{18}$cm$^{-2}$ and $T_{\mathrm{ex}} = 162.7\substack{+0.3 \\ -0.5}$K based on pixel-by-pixel Bayesian analysis of LTE synthetic methanol spectra across MM1. Intriguingly, the peak CH$_{3}$OH $T_{\mathrm{ex}}=165.5\pm0.6$ K is offset from MM1's millimetre continuum peak by $0.22''\sim880$au, and a region of elevated CH$_{3}$OH $T_{\mathrm{ex}}$ coincides with free-free VLA 5.01cm continuum, adding to the tentative evidence for a possible unresolved high-mass binary in MM1. In our VLA 1.21cm data, we report the first NH$_{3}$(3,3) maser detections towards G19.01$-$0.03, along with candidate 25GHz CH$_{3}$OH $5(2,3)-5(1,4)$ maser emission; both are spatially and kinematically coincident with 44GHz Class I CH$_{3}$OH masers in the MM1 outflow. We also report the ALMA detection of candidate 278.3GHz Class I CH$_{3}$OH maser emission towards this outflow, strengthening the connection of these three maser types to MYSO outflows.Comment: 24 pages, 15 figures, 7 tables, Accepted for publication in MNRA

ALMA observations of the Extended Green Object G19.01-0.03 : II. A massive protostar with typical chemical abundances surrounded by four low-mass prestellar core candidates

Funding: GMW acknowledges support from the UK’s Science and Technology Facilities Council (STFC) under ST/W00125X/1. CJC acknowledges support from the UK’s STFC under ST/M001296/1. PN acknowledges support by grant 618.000.001 from the Dutch Research Council (NWO) and support by the Danish National Research Foundation through the Center of Excellence "InterCat" (Grant agreement no.: DNRF150).We present a study of the physical and chemical properties of the Extended Green Object (EGO) G19.01−0.03 using sub-arcsecond angular resolution Atacama Large Millimeter/submillimeter Array (ALMA) 1.05 mm and Karl G. Jansky Very Large Array (VLA) 1.21 cm data. G19.01−0.03 MM1, the millimetre source associated with the central massive young stellar object (MYSO), appeared isolated and potentially chemically young in previous Submillimeter Array observations. In our ∼0.4″-resolution ALMA data, MM1 has four low-mass millimetre companions within 0.12 pc, all lacking maser or outflow emission, indicating they may be prestellar cores. With a rich ALMA spectrum full of complex organic molecules, MM1 does not appear chemically young, but has molecular abundances typical of high-mass hot cores in the literature. At the 1.05 mm continuum peak of MM1, N(CH3OH) = (2.22 ± 0.01) × 1018 cm−2 and Tex=162.7+0.3−0.5 K based on pixel-by-pixel Bayesian analysis of LTE synthetic methanol spectra across MM1. Intriguingly, the peak CH3OH Tex = 165.5 ± 0.6 K is offset from MM1’s millimetre continuum peak by 0.22″ ∼ 880 AU, and a region of elevated CH3OH Tex coincides with free-free VLA 5.01 cm continuum, adding to the tentative evidence for a possible unresolved high-mass binary in MM1. In our VLA 1.21 cm data, we report the first NH3(3,3) maser detections towards G19.01−0.03, along with candidate 25 GHz CH3OH 5(2, 3) − 5(1, 4) maser emission; both are spatially and kinematically coincident with 44 GHz Class I CH3OH masers in the MM1 outflow. We also report the ALMA detection of candidate 278.3 GHz Class I CH3OH maser emission towards this outflow, strengthening the connection of these three maser types to MYSO outflows.Publisher PDFPeer reviewe

Filamentary mass accretion towards the high-mass protobinary system G11.92-0.61 MM2

Funding: S.Z. is funded by the China Scholarship Council-University of St Andrews Scholarship (PhD programmes, No. 201806190010). C.J.C. acknowledges support from the University of St Andrews Restarting Research Funding Scheme (SARRF), which is funded through the SFC grant reference SFC/AN/08/020. J.D.H gratefully acknowledges financial support from the Royal Society (University Research Fellowship; URF\R1\2216.We present deep, sub-arcsecond (∼2000 AU) resolution ALMA 0.82 mm observations of the former high-mass prestellar core candidate G11.92-0.61 MM2, recently shown to be an ~500 AU-separation protobinary. Our observations show that G11.92-0.61 MM2, located in the G11.92-0.61 protocluster, lies on a filamentary structure traced by 0.82 mm continuum and N2H+(4-3) emission. The N2H+(4-3) spectra are multi-peaked, indicative of multiple velocity components along the line of sight. To analyse the gas kinematics, we performed pixel-by-pixel Gaussian decomposition of the N2H+$ spectra using SCOUSEPY and hierarchical clustering of the extracted velocity components using ACORNS. Seventy velocity- and position-coherent clusters (called "trees") are identified in the N2H+-emitting gas, with the 8 largest trees accounting for > 60 per cent  of the fitted velocity components. The primary tree, with ~20 per cent of the fitted velocity components, displays a roughly north-south velocity gradient along the filamentary structure traced by the 0.82 mm continuum. Analysing a ~0.17 pc-long substructure, we interpret its velocity gradient of ~10.5 km s-1pc-1 as tracing filamentary accretion towards MM2 and estimate a mass inflow rate of ~1.8 × 10-4 to 1.2 × 10-3 M⊙ yr-1. Based on the recent detection of a bipolar molecular outflow associated with MM2, accretion onto the protobinary is ongoing, likely fed by the larger-scale filamentary accretion flows. If 50% of the filamentary inflow reaches the protostars, each member of the protobinary would attain a mass of 8 M⊙ within ~1.6 × 105 yr, comparable to the combined timescale of the 70-μm- and mid-infrared-weak phases derived for ATLASGAL-TOP100 massive clumps using chemical clocks.Peer reviewe

A High-Resolution Survey of HI Absorption toward the Central 200 pc of the Galactic Center

We present an HI absorption survey of the central 250 pc of the Galaxy. Very Large Array (VLA) observations were made at 21 cm in the DnC and CnB configurations and have a resolution of ~15"(0.6 pc at the Galactic Center (GC) distance) and a velocity resolution of ~2.5 km/s. This study provides HI data with high spatial resolution, comparable with the many high resolution observations which have been made of GC sources over the past ten years. Here we present an overview of the HI absorption toward ~40 well-known continuum sources and a detailed comparison of the ionized, atomic and molecular components of the interstellar medium for the Sgr B, Radio Arc and Sgr C regions. In these well-known regions, the atomic gas appears to be closely correlated in both velocity and distribution to the ionized and molecular gas, indicating that it resides in photo-dissociation regions related to the HII regions in the GC. Toward the majority of the radio continuum sources, HI absorption by the 3-kpc arm is detected, constraining these sources to lie beyond a 5 kpc distance in the Galaxy.Comment: 59 pages, including 41 figures; accepted for publication in Astrophysical Journal Supplement Series in December 201

H2O masers and host environments of FU Orionis and EX Lupi type low-mass eruptive YSOs

Funding: H2020 European Research Council - 716155.The FU Orionis (FUor) and EX Lupi (EXor) type objects are rare pre-main sequence low-mass stars undergoing accretion outbursts. Maser emission is widespread and is a powerful probe of mass accretion and ejection on small scales in star forming region. However, very little is known about the overall prevalence of water masers towards FUors/Exors. We present results from our survey using the Effelsberg 100-m telescope to observe the largest sample of FUors and EXors, plus additional Gaia alerted sources (with the potential nature of being eruptive stars), a total of 51 targets, observing the 22.2 GHz H2O maser, while simultaneously covering the NH3 23 GHz.Peer reviewe

The Bolocam Galactic Plane Survey: Survey Description and Data Reduction

We present the Bolocam Galactic Plane Survey (BGPS), a 1.1 mm continuum survey at 33" effective resolution of 170 square degrees of the Galactic Plane visible from the northern hemisphere. The survey is contiguous over the range -10.5 < l < 90.5, |b| < 0.5 and encompasses 133 square degrees, including some extended regions |b| < 1.5. In addition to the contiguous region, four targeted regions in the outer Galaxy were observed: IC1396, a region towards the Perseus Arm, W3/4/5, and Gem OB1. The BGPS has detected approximately 8400 clumps over the entire area to a limiting non-uniform 1-sigma noise level in the range 11 to 53 mJy/beam in the inner Galaxy. The BGPS source catalog is presented in a companion paper (Rosolowsky et al. 2010). This paper details the survey observations and data reduction methods for the images. We discuss in detail the determination of astrometric and flux density calibration uncertainties and compare our results to the literature. Data processing algorithms that separate astronomical signals from time-variable atmospheric fluctuations in the data time-stream are presented. These algorithms reproduce the structure of the astronomical sky over a limited range of angular scales and produce artifacts in the vicinity of bright sources. Based on simulations, we find that extended emission on scales larger than about 5.9' is nearly completely attenuated (> 90%) and the linear scale at which the attenuation reaches 50% is 3.8'. Comparison with other millimeter-wave data sets implies a possible systematic offset in flux calibration, for which no cause has been discovered. This presentation serves as a companion and guide to the public data release through NASA's Infrared Processing and Analysis Center (IPAC) Infrared Science Archive (IRSA). New data releases will be provided through IPAC IRSA with any future improvements in the reduction.Comment: Accepted for publication in Astrophysical Journal Supplemen

SOFIA FORCAST photometry of 12 Extended Green Objects in the Milky Way

Funding: UK STFC grant number ST/M001296/1 (CJC).Massive young stellar objects are known to undergo an evolutionary phase in which high mass accretion rates drive strong outflows. A class of objects believed to trace this phase accurately is the Galactic Legacy Infrared Midplane Survey Extraordinaire (GLIMPSE) Extended Green Object (EGO) sample, so named for the presence of extended 4.5μm emission on size scales of∼0.1pc in Spitzer images. We have been conducting a multi-wavelength examination of a sample of 12 EGOs with distances of 1-5 kpc. In this paper, we present mid-infrared images and photometry of these EGOs obtained with the Stratospheric Observatory for Infrared Astronomy and subsequently construct spectral energy distributions (SEDs) for these sources from the near-infrared to sub-millimeter regimes using additional archival data. We compare the results from graybody models and several publicly-available software packages which produce model SEDs in the context of a single massive protostar. The models yield typical R⋆ ∼10 R⊙, T⋆ ∼103-104 K, and L⋆ ∼1−40 × 103 L⊙; the median L/M for our sample is 24.7 L⊙/M⊙. Model results rarely converge for R⋆  and T⋆, but do for L⋆, which we take to be an indication of the multiplicity and inherently clustered nature of these sources even though, typically, only a single source dominates in the mid-infrared. The median L/M value for the sample suggests that these objects may be in a transitional stage between the commonly described “IR-quiet” and “IR-bright” stages of MYSO evolution. The median Tdust for the sample is less conclusive, but suggests that these objects are either in this transitional stage or occupy the cooler (and presumably younger) part of the IR-bright stage.PostprintPeer reviewe