Circumstellar disks and planets. Science cases for next-generation optical/infrared long-baseline interferometers

We present a review of the interplay between the evolution of circumstellar disks and the formation of planets, both from the perspective of theoretical models and dedicated observations. Based on this, we identify and discuss fundamental questions concerning the formation and evolution of circumstellar disks and planets which can be addressed in the near future with optical and infrared long-baseline interferometers. Furthermore, the importance of complementary observations with long-baseline (sub)millimeter interferometers and high-sensitivity infrared observatories is outlined.Comment: 83 pages; Accepted for publication in "Astronomy and Astrophysics Review"; The final publication is available at http://www.springerlink.co

Early-stage Massive Star Formation near the Galactic Centre: Sgr C

We present near-infrared spectroscopy and 1 mm line and continuum observations of a recently identified site of high mass star formation likely to be located in the Central Molecular Zone (CMZ) near Sgr C. Located on the outskirts of the massive evolved H II region associated with Sgr C, the area is characterized by an Extended Green Object (EGO) measuring ~10'' in size (0.4 pc), whose observational characteristics suggest the presence of an embedded massive protostar driving an outflow. Our data confirm that early-stage star formation is taking place on the periphery of the Sgr C H II region, with detections of two protostellar cores and several knots of H2 and Brackett γ emission alongside a previously detected compact radio source. We calculate the cores' joint mass to be ~103 M ☉, with column densities of 1-2 × 1024 cm–2. We show the host molecular cloud to hold ~105 M ☉ of gas and dust with temperatures and column densities favorable for massive star formation to occur, however, there is no evidence of star formation outside of the EGO, indicating that the cloud is predominantly quiescent. Given its mass, density, and temperature, the cloud is comparable to other remarkable non-star-forming clouds such as G0.253 in the eastern CMZ

G11.92-0.61 MM1: A Keplerian disc around a massive young proto-O star

The formation process of massive stars is not well understood, and advancement in our understanding benefits from high resolution observations and modelling of the gas and dust surrounding individual high-mass (proto)stars. Here we report sub-arcsecond (<1550 au) resolution observations of the young massive star G11.92-0.61 MM1 with the SMA and VLA. Our 1.3 mm SMA observations reveal consistent velocity gradients in compact molecular line emission from species such as CH$_3$CN, CH$_3$OH, OCS, HNCO, H$_2$CO, DCN and CH$_3$CH$_2$CN, oriented perpendicular to the previously reported bipolar molecular outflow from MM1. Modelling of the compact gas kinematics suggests a structure undergoing rotation around the peak of the dust continuum emission. The rotational profile can be well fit by a model of a Keplerian disc, including infall, surrounding an enclosed mass of 30-60M$_{\odot}$, of which 2-3M$_{\odot}$ is attributed to the disc. From modelling the CH$_3$CN emission, we determine that two temperature components, of 150 K and 230 K, are required to adequately reproduce the spectra. Our 0.9 and 3.0cm VLA continuum data exhibit an excess above the level expected from dust emission; the full centimetre-submillimetre wavelength spectral energy distribution of MM1 is well reproduced by a model including dust emission, an unresolved hypercompact H{\i}{\i} region, and a compact ionised jet. In combination, our results suggest that MM1 is an example of a massive proto-O star forming via disc accretion, in a similar way to that of lower mass stars.European Research Council (ERC-2013-ADG DISCSIM project (Grant ID: 341137), ERC-2011-ADG ECOGAL project (Grant ID: 291227)), Science and Technology Facilities Council (Grant ID: ST/M001296/1), Royal Astronomical Society (Undergraduate Research Bursary)This is the final version of the article. It first appeared from Oxford University Press via http://dx.doi.org/10.1093/mnras/stw191

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

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 NH(4-3) emission. The NH(4-3) spectra are multipeaked, indicative of multiple velocity components along the line of sight. To analyse the gas kinematics, we performed pixel-by-pixel Gaussian decomposition of the NH 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 NH-emitting gas, with the eight 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 an 0.17 pc-long substructure, we interpret its velocity gradient of 10.5 km s pc as tracing filamentary accretion towards MM2 and estimate a mass inflow rate of to 1.2 M yr. Based on the recent detection of a bipolar molecular outflow associated with MM2, accretion on to the protobinary is ongoing, likely fed by the larger scale filamentary accretion flows. If 50 per cent of the filamentary inflow reaches the protostars, each member of the protobinary would attain a mass of 8 M within yr, comparable to the combined time-scale of the 70-μm- and mid-infrared-weak phases derived for ATLASGAL-TOP100 massive clumps using chemical clocks

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

Using the Atacama Large Millimetre/submillimeter Array (ALMA) and the Karl G. Jansky Very Large Array (VLA), we observed the Extended Green Object (EGO) G19.01-0.03 with sub-arcsec resolution from 1.05 mm to 5.01 cm wavelengths. Our 0.4 arcsec 1600 au angular resolution ALMA observations reveal a velocity gradient across the millimetre core MM1, oriented perpendicular to the previously known bipolar molecular outflow, which is consistently traced by 20 lines of 8 molecular species with a range of excitation temperatures, including complex organic molecules (COMs). Kinematic modelling shows the data are well described by models that include a disc in Keplerian rotation and infall, with an enclosed mass of 40-70 M (within a 2000 au outer radius) for a disc inclination angle of i = 40, of which 5.4-7.2 M is attributed to the disc. Our new VLA observations show that the 6.7 GHz Class II methanol masers associated with MM1 fo a partial ellipse, consistent with an inclined ring, with a velocity gradient consistent with that of the theal gas. The disc-to-star mass ratio suggests the disc is likely to be unstable and may be fragmenting into as-yet-undetected low-mass stellar companions. Modelling the centimetre-millimetre spectral energy distribution of MM1 shows the ALMA 1.05 mm continuum emission is dominated by dust, whilst a free-free component, interpreted as a hypercompact H ii region, is required to explain the VLA 5 cm emission. The high enclosed mass derived for a source with a moderate bolometric luminosity (104L) suggests that the MM1 disc may feed an unresolved high-mass binary system

Dual cometary HII regions in DR 21: Bow shocks or champagne flows?

[[abstract]]The DR 21 massive star-forming region contains two cometary H II regions, aligned nearly perpendicular to each other on the sky. This others a unique opportunity to discriminate among models of cometary H II regions. We present hydrogen recombination and ammonia line observations of DR 21 made with the Very Large Array. The velocity of the molecular gas, measured from NH3 emission and absorption lines, is constant to within +/-1 km s(-1) across the region. However, the radial velocity of the ionized material, measured from hydrogen recombination lines, differs by approximate to9 km s(-1) between the &quot;heads'' of the two cometary H II regions and by up to similar to7 km s(-1) from that of the molecular gas. These findings indicate a supersonic velocity difference between the compact heads of the cometary regions and between each head and the ambient molecular material. This suggests that the observed cometary morphologies are created largely by the motion of wind-blowing, ionizing stars through the molecular cloud, as in a bow shock model.[[fileno]]2010504010005[[department]]天文

VLA observations of nine extended green objects in the Milky Way : ubiquitous weak, compact continuum emission, and multi-epoch emission from methanol, water, and ammonia masers

Support for this work was provided by the NSF through the Grote Reber Fellowship Program administered by Associated Universities, Inc./National Radio Astronomy Observatory. Support for this work was provided by the NSF through award SOSP18A-007 from the NRAO. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under agreement by the Associated Universities, Inc.We have observed a sample of nine Extended Green Objects (EGOs) at 1.3 and 5 cm with the Very Large Array (VLA) with subarcsecond resolution and ∼7–14 μJy beam−1-sensitivities in order to characterize centimeter continuum emission as it first appears in these massive protoclusters. We find an EGO-associated continuum emission—within 1'' of the extended 4.5 μm emission—in every field, which is typically faint (order 101–102 μJy) and compact (unresolved at 0''.3–0''.5). The derived spectral indices of our 36 total detections are consistent with a wide array of physical processes, including both non-thermal (19% of detections) and thermal free–free processes (e.g., ionized jets and compact H ii regions, 78% of sample) and warm dust (1 source). We also find an EGO-associated 6.7 GHz CH3OH and 22 GHz H2O maser emission in 100% of the sample and a NH3 (3,3) masers in ∼45%; we do not detect any NH3 (6,6) masers at ∼5.6 mJy beam−1 sensitivity. We find statistically-significant correlations between Lradio and Lbol at two physical scales and three frequencies, consistent with thermal emission from ionized jets, but no correlation between LH20 and Lradio for our sample. From these data, we conclude that EGOs likely host multiple different centimeter continuum-producing processes simultaneously. Additionally, at our ∼1000 au resolution, we find that all EGOs except G18.89−0.47 contain 1 ∼ 2 massive sources based on the presence of CH3OH maser groups, which is consistent with our previous work suggesting that these are typical massive protoclusters, in which only one to a few of the young stellar objects are massive.Peer reviewe

Dual cometary HII regions in DR 21: Bow shocks or champagne flows?

The DR 21 massive star-forming region contains two cometary H II regions, aligned nearly perpendicular to each other on the sky. This others a unique opportunity to discriminate among models of cometary H II regions. We present hydrogen recombination and ammonia line observations of DR 21 made with the Very Large Array. The velocity of the molecular gas, measured from NH3 emission and absorption lines, is constant to within +/-1 km s(-1) across the region. However, the radial velocity of the ionized material, measured from hydrogen recombination lines, differs by approximate to9 km s(-1) between the "heads'' of the two cometary H II regions and by up to similar to7 km s(-1) from that of the molecular gas. These findings indicate a supersonic velocity difference between the compact heads of the cometary regions and between each head and the ambient molecular material. This suggests that the observed cometary morphologies are created largely by the motion of wind-blowing, ionizing stars through the molecular cloud, as in a bow shock model.</p

G11.92-0.61 MM 1: A Fragmented Keplerian Disk Surrounding a Proto-O Star

Abstract We present high-resolution (∼300 au) Atacama Large Millimeter/submillimeter Array observations of the massive young stellar object G11.92–0.61 MM 1. We resolve the immediate circumstellar environment of MM 1 in 1.3 mm continuum emission and CH3CN emission for the first time. The object divides into two main sources—MM 1a, which is the source of a bipolar molecular outflow, and MM 1b, located 0.″57 (1920 au) to the southeast. The main component of MM 1a is an elongated continuum structure, perpendicular to the bipolar outflow, with a size of 0.″141 × 0.″050 (480 × 170 au). The gas kinematics toward MM 1a probed via CH3CN trace a variety of scales. The lower energy J = 12–11 K = 3 line traces extended, rotating gas within the outflow cavity, while the v8 = 1 line shows a clearly resolved Keplerian rotation signature. Analysis of the gas kinematics and dust emission shows that the total enclosed mass in MM 1a is 40 ± 5 M ⊙ (where between 2.2 and 5.8 M ⊙ is attributed to the disk), while MM 1b is &lt;0.6 M ⊙. The extreme mass ratio and orbital properties of MM 1a and MM 1b suggest that MM 1b is one of the first observed examples of the formation of a binary star via disk fragmentation around a massive young (proto)star.</jats:p