27 research outputs found
Stellar masses and disk properties of Lupus young stellar objects traced by velocity-aligned stacked ALMA 13CO and C18O spectra
In recent ALMA surveys, the gas distributions and velocity structures of most
of the protoplanetary disks can still not be imaged at high S/N due to the
short integration time. In this work, we re-analyzed the ALMA 13CO (3-2) and
C18O (3-2) data of 88 young stellar objects in Lupus with the velocity-aligned
stacking method to enhance S/N and to study the kinematics and disk properties
traced by molecular lines. This method aligns spectra at different positions in
a disk based on the projected Keplerian velocities at their positions and then
stacks them. This method enhances the S/N ratios of molecular-line data and
allows us to obtain better detections and to constrain dynamical stellar masses
and disk orientations. We obtain 13CO detections in 41 disks and C18O
detections in 18 disks with 11 new detections in 13CO and 9 new detections in
C18O after applying the method. We estimate the disk orientations and the
dynamical stellar masses from the 13CO data. Our estimated dynamical stellar
masses correlate with the spectroscopic stellar masses, and in a subsample of
16 sources, where the inclination angles are better constrained, the two masses
are in a good agreement within the uncertainties and with a mean difference of
0.15 Msun. With more detections of fainter disks, our results show that high
gas masses derived from the 13CO and C18O lines tend to be associated with high
dust masses estimated from the continuum emission. Nevertheless, the scatter is
large (0.9 dex), implying large uncertainties in deriving the disk gas mass
from the line fluxes. We find that with such large uncertainties it is expected
that there is no correlation between the disk gas mass and the mass accretion
rate with the current data. Deeper observations to detect disks with gas masses
<1E-5 Msun in molecular lines are needed to investigate the correlation between
the disk gas mass and the mass accretion rate.Comment: Submitted to A&
Protoplanetary disk masses from CO isotopologues line emission
One of the methods for deriving disk masses relies on direct observations of
the gas, whose bulk mass is in the outer cold (K) regions. This
zone can be well traced by rotational lines of less abundant CO isotopologues,
that probe the gas down to the midplane. The total CO gas mass is then obtained
with the isotopologue ratios taken to be constant at the elemental isotope
values found in the local ISM. This approach is however imprecise, because
isotope selective processes are ignored. The aim of this work is an
isotopologue selective treatment of CO isotopologues, in order to obtain a more
accurate determination of disk masses. The isotope-selective photodissociation,
the main process controlling the abundances of CO isotopologues in the
CO-emissive layer, is properly treated for the first time in a full disk model
(DALI, Bruderer et al. 2012; Bruderer 2013). The chemistry, thermal balance,
line and continuum radiative transfer are all considered together with a
chemical network that treats CO, CO, CO, isotopes of all
included atoms, and molecules, as independent species. Isotope selective
processes lead to regions in the disk where the isotopologues abundance ratios
are considerably different from the elemental ratio. The results of this work
show that considering CO isotopologue ratios as constants can lead to an
underestimate of disk masses by up to almost two orders of magnitude if grains
have grown to larger sizes. This may explain observed discrepancies in mass
determinations from different tracers. The dependence of the various
isotopologues emission on stellar and disk parameters is investigated.
Including CO isotope selective processes is crucial to determine the gas mass
of the disk accurately (through ALMA observations) and thus to provide the
amount of gas which may eventually form planets or change the dynamics of
forming planetary systems.Comment: Accepted for publication in A&A, 16 pages, 10 figures, 4 table
Probing the protoplanetary disk gas surface density distribution with CO emission
It is key to constrain the gas surface density distribution, Sigma_gas, as
function of disk radius in protoplanetary disks. In this work we investigate if
spatially resolved observations of rarer CO isotopologues may be good tracers
of Sigma_gas. Physical-chemical models with different input Sigma_gas(R) are
run. The input disk surface density profiles are compared with the simulated
13CO intensity radial profiles to check if and where the two follow each other.
There is always an intermediate region in the disk where the slope of the 13CO
radial emission profile and Sigma_gas(R) coincide. At small radii the line
radial profile underestimates Sigma_gas, as 13CO emission becomes optically
thick. The same happens at large radii where the column densities become too
low and 13CO is not able to efficiently self-shield. If the gas surface density
profile is a simple power-law of the radius, the input power-law index can be
retrieved within 20% uncertainty if one choses the proper radial range. If
instead Sigma_gas(R) follows the self-similar solution for a viscously evolving
disk, retrieving the input power-law index becomes challenging, in particular
for small disks. Nevertheless, it is found that the power-law index can be in
any case reliably fitted at a given line intensity contour around 6 K km/s, and
this produces a practical method to constrain the slope of Sigma_gas(R).
Application of such a method is shown in the case study of the TW Hya disk.
Spatially resolved 13CO line radial profiles are promising to probe the disk
surface density distribution, as they directly trace Sigma_gas(R)profile at
radii well resolvable by ALMA. There, chemical processes like freeze-out and
isotope selective photodissociation do not affect the emission, and, assuming
that the volatile carbon does not change with radius, no chemical model is
needed when interpreting the observations.Comment: 14 pages, 10 figures, A&A accepte
An ALMA Survey of Protoplanetary Disks in the Orionis Cluster
The Orionis cluster is important for studying protoplanetary disk
evolution, as its intermediate age (3-5 Myr) is comparable to the median
disk lifetime. We use ALMA to conduct a high-sensitivity survey of dust and gas
in 92 protoplanetary disks around Orionis members with
. Our observations cover the 1.33 mm continuum
and several CO lines: out of 92 sources, we detect 37 in the mm
continuum and six in CO, three in CO, and none in CO.
Using the continuum emission to estimate dust mass, we find only 11 disks with
, indicating that after only a few Myr of
evolution most disks lack sufficient dust to form giant planet cores. Stacking
the individually undetected continuum sources limits their average dust mass to
5 lower than that of the faintest detected disk, supporting theoretical
models that indicate rapid dissipation once disk clearing begins. Comparing the
protoplanetary disk population in Orionis to those of other
star-forming regions supports the steady decline in average dust mass and the
steepening of the - relation with age; studying these
evolutionary trends can inform the relative importance of different disk
processes during key eras of planet formation. External photoevaporation from
the central O9 star is influencing disk evolution throughout the region: dust
masses clearly decline with decreasing separation from the photoionizing
source, and the handful of CO detections exist at projected separations
pc. Collectively, our findings indicate that giant planet formation is
inherently rare and/or well underway by a few Myr of age.Comment: 16 pages, 9 figures; published in AJ; The full machine readable
tables can be obtained by downloading and extracting the gzipped tar source
file listed under "Other formats.
Probing UV-sensitive Pathways for CN and HCN Formation in Protoplanetary Disks with the Hubble Space Telescope
The UV radiation field is a critical regulator of gas-phase chemistry in surface layers of disks around young stars. In an effort to understand the relationship between photocatalyzing UV radiation fields and gas emission observed at infrared and submillimeter wavelengths, we present an analysis of new and archival Hubble Space Telescope (HST), Spitzer, ALMA, IRAM, and SMA data for five targets in the Lupus cloud complex and 14 systems in Taurus-Auriga. The HST spectra were used to measure Lyα and far-UV (FUV) continuum fluxes reaching the disk surface, which are responsible for dissociating relevant molecular species (e.g., HCN, Nâ). Semi-forbidden C II] λ2325 and UV-fluorescent Hâ emission were also measured to constrain inner disk populations of Câș and vibrationally excited H2. We find a significant positive correlation between 14 ÎŒm HCN emission and fluxes from the FUV continuum and C II] λ2325, consistent with model predictions requiring Nâ photodissociation and carbon ionization to trigger the main CN/HCN formation pathways. We also report significant negative correlations between submillimeter CN emission and both C II] and FUV continuum fluxes, implying that CN is also more readily dissociated in disks with stronger FUV irradiation. No clear relationships are detected between either CN or HCN and Lyα or UV-Hâ emission. This is attributed to the spatial stratification of the various molecular species, which span several vertical layers and radii across the inner and outer disk. We expect that future observations with the James Webb Space Telescope will build on this work by enabling more sensitive IR surveys than were possible with Spitzer
Resolved gas cavities in transitional disks inferred from CO isotopologues with ALMA
Transitional disks around young stars are promising candidates to look for
recently formed, embedded planets. Planet-disk interaction models predict that
planets clear a gap in the gas while trapping dust at larger radii. Other
physical mechanisms could be responsible for cavities as well. Previous
observations have revealed that gas is still present inside these cavities, but
the spatial distribution of this gas remains uncertain. We present high spatial
resolution observations with the Atacama Large Millimeter/submillimeter Array
(ALMA) of 13CO and C18O lines of four well-studied transitional disks. The
observations are used to set constraints on the gas surface density,
specifically cavity size and density drop inside the cavity. The
physical-chemical model DALI is used to analyze the gas images of SR21,
HD135344B, DoAr44 and IRS48. The main parameters of interest are the size,
depth and shape of the gas cavity. CO isotope-selective photodissociation is
included to properly constrain the surface density in the outer disk from C18O
emission. The gas cavities are up to 3 times smaller than those of the dust in
all four disks. Model fits indicate that the surface density inside the gas
cavities decreases by a factor of 100-10000 compared with the surface density
profile derived from the outer disk. A comparison with an analytical model of
gap depths by planet-disk interaction shows that the disk viscosities are
likely low, with a<1E-3 for planet masses <10 MJup. The resolved measurements
of the gas and dust in transition disk cavities support the predictions of
models that describe how planet-disk interactions sculpt gas disk structures
and influence the evolution of dust grains. These observed structures strongly
suggest the presence of giant planetary companions in transition disk cavities,
although at smaller orbital radii than is typically indicated from the dust
cavity radii alone.Comment: Accepted by A&A; version after language-editin
New insights into the nature of transition disks from a complete disk survey of the Lupus star forming region
Transition disks with large dust cavities around young stars are promising
targets for studying planet formation. Previous studies have revealed the
presence of gas cavities inside the dust cavities hinting at recently formed,
giant planets. However, many of these studies are biased towards the brightest
disks in the nearby star forming regions, and it is not possible to derive
reliable statistics that can be compared with exoplanet populations. We present
the analysis of 11 transition disks with large cavities (>20 AU radius) from a
complete disk survey of the Lupus star forming region, using ALMA Band 7
observations at 0.3" (22-30 AU radius) resolution of the 345 GHz continuum,
13CO and C18O 3-2 observations and the Spectral Energy Distribution of each
source. Gas and dust surface density profiles are derived using the
physical-chemical modeling code DALI. This is the first study of transition
disks of large cavities within a complete disk survey within a star forming
region. The dust cavity sizes range from 20-90 AU radius and in three cases, a
gas cavity is resolved as well. The deep drops in gas density and large dust
cavity sizes are consistent with clearing by giant planets. The fraction of
transition disks with large cavities in Lupus is ~11%, which is inconsistent
with exoplanet population studies of giant planets at wide orbits. Furthermore,
we present a hypothesis of an evolutionary path for large massive disks
evolving into transition disks with large cavities.Comment: 29 pages, 15 figures, Accepted by Ap
Millimeter emission in photoevaporating disks is determined by early substructures
[abridged]Photoevaporation and dust-trapping are individually considered to
be important mechanisms in the evolution and morphology of protoplanetary
disks. We studied how the presence of early substructures affects the evolution
of the dust distribution and flux in the millimeter continuum of disks that are
undergoing photoevaporative dispersal. We also tested if the predicted
properties resemble those observed in the population of transition disks. We
used the numerical code Dustpy to simulate disk evolution considering gas
accretion, dust growth, dust-trapping at substructures, and mass loss due to
X-ray and EUV (XEUV) photoevaporation and dust entrainment. Then, we compared
how the dust mass and millimeter flux evolve for different disk models. We find
that, during photoevaporative dispersal, disks with primordial substructures
retain more dust and are brighter in the millimeter continuum than disks
without early substructures, regardless of the photoevaporative cavity size.
Once the photoevaporative cavity opens, the estimated fluxes for the disk
models that are initially structured are comparable to those found in the
bright transition disk population (), while
the disk models that are initially smooth have fluxes comparable to the
transition disks from the faint population (), suggesting a link between each model and population. Our models
indicate that the efficiency of the dust trapping determines the millimeter
flux of the disk, while the gas loss due to photoevaporation controls the
formation and expansion of a cavity, decoupling the mechanisms responsible for
each feature. In consequence, even a planet with a mass comparable to Saturn
could trap enough dust to reproduce the millimeter emission of a bright
transition disk, while its cavity size is independently driven by
photoevaporative dispersal.Comment: Accepted for publication in A&
Probing UV-sensitive Pathways for CN and HCN Formation in Protoplanetary Disks with the Hubble Space Telescope
The UV radiation field is a critical regulator of gas-phase chemistry in surface layers of disks around young stars. In an effort to understand the relationship between photocatalyzing UV radiation fields and gas emission observed at infrared and submillimeter wavelengths, we present an analysis of new and archival Hubble Space Telescope (HST), Spitzer, ALMA, IRAM, and SMA data for five targets in the Lupus cloud complex and 14 systems in Taurus-Auriga. The HST spectra were used to measure Lyα and far-UV (FUV) continuum fluxes reaching the disk surface, which are responsible for dissociating relevant molecular species (e.g., HCN, Nâ). Semi-forbidden C II] λ2325 and UV-fluorescent Hâ emission were also measured to constrain inner disk populations of Câș and vibrationally excited H2. We find a significant positive correlation between 14 ÎŒm HCN emission and fluxes from the FUV continuum and C II] λ2325, consistent with model predictions requiring Nâ photodissociation and carbon ionization to trigger the main CN/HCN formation pathways. We also report significant negative correlations between submillimeter CN emission and both C II] and FUV continuum fluxes, implying that CN is also more readily dissociated in disks with stronger FUV irradiation. No clear relationships are detected between either CN or HCN and Lyα or UV-Hâ emission. This is attributed to the spatial stratification of the various molecular species, which span several vertical layers and radii across the inner and outer disk. We expect that future observations with the James Webb Space Telescope will build on this work by enabling more sensitive IR surveys than were possible with Spitzer