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Register a new userChasing discs around O-type (proto)stars - ALMA evidence for an SiO disc and disc wind from G17.64+0.16
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We present high angular resolution 0.2 arcsec continuum and molecular emission line Atacama Large Millimeter/sub-millimeter Array (ALMA) observations of G17.64+0.16 in Band 6 (220GHz) taken as part of a campaign in search of circumstellar discs around (proto)-O-stars. At a resolution of 400au the main continuum core is essentially unresolved and isolated from other strong and compact emission peaks. At a resolution of 400au the main continuum core is essentially unresolved and isolated from other strong and compact emission peaks. We detect SiO (5-4) emission that is marginally resolved and elongated in a direction perpendicular to the large-scale outflow seen in the 13CO (2-1) line using the main ALMA array in conjunction with the Atacama Compact Array (ACA). Morphologically, the SiO appears to represent a disc-like structure. Using parametric models we show that the position-velocity profile of the SiO is consistent with the Keplerian rotation of a disc around an object between 10-30Mo in mass, only if there is also radial expansion from a separate structure. The radial motion component can be interpreted as a disc wind from the disc surface. Models with a central stellar object mass between 20 and 30Mo are the most consistent with the stellar luminosity (100000 Lo) and indicative of an O-type star. The H30a millimetre recombination line (231.9GHz) is also detected, but spatially unresolved, and is indicative of a very compact, hot, ionised region co-spatial with the dust continuum core. Accounting for all observables, we suggest that G17.64 is consistent with a O-type young stellar object in the final stages of protostellar assembly, driving a wind, but that has not yet developed into a compact HII region. The existance and detection of the disc in G17.64 is likely related to its isolated and possibly more evolved nature, traits which may underpin discs in similar sources.
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We present the highest angular resolution (20x15mas - 44x33au) Atacama Large Millimeter/sub-millimeter Array (ALMA) observations currently possible of the proto-O-star G17.64+0.16 in Band 6. The Cycle 5 observations with baselines out to 16km probes scales <50au and reveal the rotating disc around G17.64+0.16, a massive forming O-type star. The disc has a ring-like enhancement in the dust emission, especially visible as arc structures to the north and south. The Keplerian kinematics are most prominently seen in the vibrationally excited water line, H2O (Eu=3461.9K). The mass of the central source found by modelling the Keplerian rotation is consistent with 45+/-10Mo. The H30alpha (231.9GHz) radio-recombination line and the SiO (5-4) molecular line were detected at up to the 10 sigma$ level. The estimated disc mass is 0.6-2.6Mo under the optically thin assumption. Analysis of the Toomre Q parameter, in the optically thin regime, indicates that the disc stability is highly dependent on temperature. The disc currently appears stable for temperatures >150K, this does not preclude that the substructures formed earlier through disc fragmentation.
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.
Scattered light high-resolution imaging of the proto-planetary disc orbiting HD100453 shows two symmetric spiral arms, possibly launched by an external stellar companion. In this paper we present new, sensitive high-resolution ($sim$30 mas) Band 7 ALMA observations of this source. This is the first source where we find counterparts in the sub-mm continuum to both scattered light spirals. The CO J=3-2 emission line also shows two spiral arms; in this case they can be traced over a more extended radial range, indicating that the southern spiral arm connects to the companion position. This is clear evidence that the companion is responsible for launching the spirals. The pitch angle of the sub-millimeter continuum spirals ($sim 6 ^{circ}$) is lower than the one in scattered light ($sim 16 ^{circ}$). We show that hydrodynamical simulations of binary-disc interaction can account for the difference in pitch angle only if one takes into account that the midplane is colder than the upper layers of the disc, as expected for the case of externally irradiated discs.
We present a kinematical study of 314 RR~Lyrae stars in the solar neighbourhood using the publicly available photometric, spectroscopic, and {it Gaia} DR2 astrometric data to explore their distribution in the Milky Way. We report an overdensity of 22 RR~Lyrae stars in the solar neighbourhood at a pericenter distance of between 5--9,kpc from the Galactic center. Their orbital parameters and their chemistry indicate that these 22 variables share the kinematics and the [Fe/H] values of the Galactic disc, with an average metallicity and tangential velocity of [Fe/H]=$-0.60$,dex and $v_{theta} = 241$,km,s$^{-1}$, respectively. From the distribution of the Galactocentric spherical velocity components, we find that these 22 disc-like RR~Lyrae variables are not consistent with the {it Gaia} Sausage ({it Gaia}-Enceladus), unlike almost half of the local RR~Lyrae stars. Chemical information from the literature shows that the majority of the selected pericenter peak RR~Lyrae variables are $alpha$-poor, a property shared by typically much younger stars in the thin disc. Using the available photometry we rule out a possible misclassification with the known classical and anomalous Cepheids. The similar kinematic, chemical, and pulsation properties of these disc RR~Lyrae stars suggest they share a common origin. In contrast, we find the RR~Lyrae stars associated with the {it Gaia}-Enceladus based on their kinematics and chemical composition show a considerable metallicity spread in the old population ($sim$~1,dex).
Direct imaging searches have revealed many very low-mass objects, including a small number of planetary mass objects, as wide-orbit companions to young stars. The formation mechanism of these objects remains uncertain. In this paper we present the predictions of the disc fragmentation model regarding the properties of the discs around such low-mass objects. We find that the discs around objects that have formed by fragmentation in discs hosted by Sun-like stars (referred to as parent discs and parent stars) are more massive than expected from the ${M}_{rm disc}-M_*$ relation (which is derived for stars with masses $M_*>0.2 {rm M}_{odot}$). Accordingly, the accretion rates onto these objects are also higher than expected from the $dot{M}_*-M_*$ relation. Moreover there is no significant correlation between the mass of the brown dwarf or planet with the mass of its disc nor with the accretion rate from the disc onto it. The discs around objects that form by disc fragmentation have larger than expected masses as they accrete gas from the disc of their parent star during the first few kyr after they form. The amount of gas that they accrete and therefore their mass depend on how they move in their parent disc and how they interact with it. Observations of disc masses and accretion rates onto very low-mass objects are consistent with the predictions of the disc fragmentation model. Future observations (e.g. by ALMA) of disc masses and accretion rates onto substellar objects that have even lower masses (young planets and young, low-mass brown dwarfs), where the scaling relations predicted by the disc fragmentation model diverge significantly from the corresponding relations established for higher-mass stars, will test the predictions of this model.
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