46 research outputs found
Importance of source structure on complex organics emission III. Effect of disks around massive protostars
Complex organic molecules are only detected toward a fraction of high-mass
protostars. The goal of this work is to investigate whether high-mass disks can
explain the lack of methanol emission from some massive protostellar systems.
We consider an envelope-only and an envelope-plus-disk model and use RADMC-3D
to calculate the methanol emission. High and low millimeter (mm) opacity dust
are considered for both models separately and the methanol abundance is
parameterized. Viscous heating is included due to the high accretion rates of
these objects in the disk. In contrast with low-mass protostars, the presence
of a disk does not significantly affect the temperature structure and methanol
emission. The shadowing effect of the disk is not as important for high-mass
objects and the disk mid-plane is hot because of viscous heating, which is
effective due to the high accretion rates. Consistent with observations of
infrared absorption lines toward high-mass protostars, we find a vertical
temperature inversion, i.e. higher temperatures in the disk mid-plane than the
disk surface, at radii < 50au for the models with L and
large mm opacity dust as long as the envelope mass is >550 M. The
large observed scatter in methanol emission from massive protostars can be
mostly explained toward lower luminosity objects with the envelope-plus-disk
models including low and high mm opacity dust. The methanol emission variation
toward sources with high luminosities cannot be explained by models with or
without a disk. However, the of these objects suggest that they could be
associated with hypercompact/ultracompact HII regions. Therefore, the low
methanol emission toward the high-luminosity sources can be explained by them
hosting an HII region where methanol is absent.Comment: 25 pages, 24 figures, Accepted for publication in A&
Constraining turbulence in protoplanetary discs using the gap contrast: an application to the DSHARP sample
Constraining the strength of gas turbulence in protoplanetary discs is an
open problem that has relevant implications for the physics of gas accretion
and planet formation. In this work, we gauge the amount of turbulence in 6 of
the discs observed in the DSHARP programme by indirectly measuring the vertical
distribution of their dust component. We employ the differences in the gap
contrasts observed along the major and the minor axes due to projection
effects, and build a radiative transfer model to reproduce these features for
different values of the dust scale heights. We find that (a) the scale heights
that yield a better agreement with data are generally low ( AU at a
radial distance of AU), and in almost all cases we are only able to place
upper limits on their exact values; these conclusions imply (assuming an
average Stokes number of ) low turbulence levels of
; (b) for the 9 other systems we
considered out of the DSHARP sample, our method yields no significant
constraints on the disc vertical structure; we conclude that this is because
these discs have either a low inclination or gaps that are not deep enough.
Based on our analysis we provide an empirical criterion to assess whether a
given disc is suitable to measure the vertical scale height.Comment: Accepted for publication in MNRAS. 13 pages + appendix, 12 figure
Demographics of young stars and their protoplanetary disks: lessons learned on disk evolution and its connection to planet formation
Since Protostars and Planets VI (PPVI), our knowledge of the global
properties of protoplanetary and debris disks, as well as of young stars, has
dramatically improved. At the time of PPVI, mm-observations and optical to
near-infrared spectroscopic surveys were largely limited to the Taurus
star-forming region, especially of its most massive disk and stellar
population. Now, near-complete surveys of multiple star-forming regions cover
both spectroscopy of young stars and mm interferometry of their protoplanetary
disks. This provides an unprecedented statistical sample of stellar masses and
mass accretion rates, as well as disk masses and radii, for almost 1000 young
stellar objects within 300 pc from us, while also sampling different
evolutionary stages, ages, and environments. At the same time, surveys of
debris disks are revealing the bulk properties of this class of more evolved
objects. This chapter reviews the statistics of these measured global star and
disk properties and discusses their constraints on theoretical models
describing global disk evolution. Our comparisons of observations to
theoretical model predictions extends beyond the traditional viscous evolution
framework to include analytical descriptions of magnetic wind effects. Finally,
we discuss how recent observational results can provide a framework for models
of planet population synthesis and planet formation.Comment: Review Chapter for Protostars and Planets VII, Editors: Shu-ichiro
Inutsuka, Yuri Aikawa, Takayuki Muto, Kengo Tomida, and Motohide Tamura.
Accepted version after interaction with the referees and before community
feedback. 21 pages (24 with references), 8 figures. Data table available at
http://ppvii.org/chapter/15
The distribution of accretion rates as a diagnostic of protoplanetary disc evolution
We show that the distribution of observed accretion rates is a powerful
diagnostic of protoplanetary disc physics. Accretion due to turbulent
("viscous") transport of angular momentum results in a fundamentally different
distribution of accretion rates than accretion driven by magnetised disc winds.
We find that a homogeneous sample of 300 observed accretion rates
would be sufficient to distinguish between these two mechanisms of disc
accretion at high confidence, even for pessimistic assumptions. Current samples
of T Tauri star accretion rates are not this large, and also suffer from
significant inhomogeneity, so both viscous and wind-driven models are broadly
consistent with the existing observations. If accretion is viscous, the
observed accretion rates require low rates of disc photoevaporation
(Myr). Uniform, homogeneous surveys of
stellar accretion rates can therefore provide a clear answer to the
long-standing question of how protoplanetary discs accrete.Comment: 10 pages, 8 figures. Accepted for publication in MNRA
Protoplanetary disc evolution affected by star-disc interactions in young stellar clusters
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. © 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.Most stars form in a clustered environment. Therefore, it is important to assess how this environment influences the evolution of protoplanetary discs around young stars. In turn, this affects their ability to produce planets and ultimately life. We present here for the first time 3D smoothed particle hydrodynamics/N-body simulations that include both the hydrodynamical evolution of the discs around their natal stars, as well as the dynamics of the stars themselves. The discs are viscously evolving, accreting mass on to the central star and spreading. We find penetrating encounters to be very destructive for the discs as in previous studies, although the frequency of such encounters is low. We also find, however, that encounter influence the disc radii more strongly than other disc properties such as the disc mass. The disc sizes are set by the competition between viscous spreading and the disruptive effect of encounters. As discs spread, encounters become more and more important. In the regime of rapid spreading, encounters simply truncate the discs, stripping the outer portions. In the opposite regime, we find that the effect of many distant encounters is able to limit the disc size. Finally, we predict from our simulations that disc sizes are limited by encounters at stellar densities exceeding ∼2–3 × 103 pc−2.Peer reviewe
MHD disc winds can reproduce fast disc dispersal and the correlation between accretion rate and disc mass in Lupus
Stars and planetary system
Synthetic populations of protoplanetary disks. Impact of magnetic fields and radiative transfer
Protostellar disks are the product of angular momentum conservation during
the protostellar collapse. Understanding their formation is crucial because
they are the birthplace of planets and because their formation is tightly
related to star formation. Unfortunately, the initial properties of Class 0
disks and their evolution are still poorly constrained observationally and
theoretically. We aim to better understand the mechanisms that set the
statistics of disk properties as well as to study their formation in massive
protostellar clumps. We also want to provide the community with synthetic disk
populations to better interpret young disk observations. We use the ramses code
to model star and disk formation in massive protostellar clumps with MHD
including the effect of ambipolar diffusion and RT including the stellar
radiative feedback. Those simulations, resolved up to the astronomical unit
scale, allow to investigate the formation of disk populations. Magnetic fields
play a crucial role in disk formation. A weaker initial field leads to larger
and massive disks and weakens the stellar radiative feedback by increasing
fragmentation. We find that ambipolar diffusion impacts disk and star formation
and leads to very different disk magnetic properties. The stellar radiative
feedback also have a strong influence, increasing the temperature and reducing
fragmentation. Comparing our disk populations with observations reveals that
our models with a mass-to-flux ratio of 10 seems to better reproduce observed
disk sizes. This also sheds light on a tension between models and observations
for the disk masses. The clump properties and physical modeling impact disk
populations significantly. The tension between observations and models for disk
mass estimates is critical to solve with synthetic observations in future
years, in particular for our comprehension of planet formation.Comment: 20 pages, 15 figures, accepted for publication in Astronomy &
Astrophysic
PENELLOPE:III. the peculiar accretion variability of XX Cha and its impact on the observed spread of accretion rates
The processes regulating protoplanetary disk evolution are constrained by studying how mass accretion rates scale with stellar and disk properties. The spread in these relations can be used as a constraint to the models of disk evolution, but only if the impact of accretion variability is correctly accounted for. While the effect of variability might be substantial in the embedded phases of star formation, it is often considered limited at later stages. Here we report on the observed large variation in the accretion rate for one target, XX Cha, and we discuss the impact on population studies of classical T Tauri stars. The mass accretion rate determined by fitting the UV-to-near-infrared spectrum in recent X-shooter observations is compared with the one measured with the same instrument 11 years before. XX Cha displays an accretion variability of almost 2 dex between 2010 and 2021. Although the timescales on which this variability happens are uncertain, XX Cha displays an extreme accretion variability for a classical T Tauri star. If such behavior is common among classical T Tauri stars, possibly on longer timescales than previously probed, it could be relevant for discussing the disk evolution models constrained by the observed spread in accretion rates. Finally, we remark that previous studies of accretion variability based on spectral lines may have underestimated the variability of some targets
High gas-to-dust size ratio indicating efficient radial drift in the mm-faint CX Tauri disk
The large majority of protoplanetary disks have very compact (AU) continuum emission at mm wavelengths. However, high angular resolution observations that resolve these small disks are still lacking, due to their intrinsically fainter emission compared with large bright disks. In this letter, we present mm ALMA data of the faint (mJy) disk orbiting the TTauri star CX Tau at a resolution of mas, AU in diameter. The mm-dust disk is compact, with a 68 enclosing flux radius of 14AU, and the intensity profile exhibits a sharp drop between 10-20AU, and a shallow tail between 20-40AU. No clear signatures of substructure in the dust continuum are observed, down to the same sensitivity level of the DSHARP large program. However, the angular resolution does not allow to detect substructures at a scale of the disk aspect ratio in the inner regions. The radial intensity profile resembles well the inner regions of more extended disks imaged at the same resolution in DSHARP, but with no rings present in the outer disk. No inner cavity is detected, even though the disk has been classified as a transition disk from the spectral energy distribution in the near infrared. The emission of CO is much more extended, with a 68 enclosing flux radius of 75AU. The large difference of the mm dust and gas extents () strongly points to the occurrence of radial drift, and matches well the predictions of theoretical models