Abstract
Much is known about the processes driving accretion from protoplanetary disks onto low-mass pre-main-sequence stars (T Tauri stars). Nevertheless, it is unclear how accretion stops. To ...determine the accretion properties and their relation to stellar properties and gain insight into the last stages of accretion, we present a detailed analysis of 24 low and possible accretors, previously identified using the He
i
λ
10830 line. We model moderate-resolution H
α
profiles of these stars using magnetospheric accretion flow models that account for the chromospheric contribution at the line center. Based on parameters derived from the fits of 20 stars that can be reproduced with the models, we find a power-law relation between the disk truncation radius and the mass accretion rate consistent with predictions from theory and simulations. Comparing the corotation and truncation radii, we find that most of our targets are accreting in the unstable regime and rule out the propeller as the main process stopping accretion. For the truncation radius to be the same as the magnetic radius, the dipole magnetic field and/or the efficiency parameter
ξ
need to be smaller than previously determined, suggesting that higher-order fields dominate in low accretion rates. Lastly, we determine that the lowest accretion rates that can be detected by H
α
line modeling are (1–3) × 10
−11
M
☉
yr
−1
for M3 stars and (3–5) × 10
−11
M
☉
yr
−1
for K5 stars. These limits are lower than the observed accretion rates in our sample, suggesting that we have reached a physical lower limit. This limit,
M
̇
∼
10
−
10
M
☉
yr
−
1
, is consistent with EUV-dominated photoevaporation.
The PDS 70 system has been subject to many studies in the past year following the discovery of two accreting planets in the gap of its circumstellar disk. Nevertheless, the mass accretion rate onto ...the star is still not well known. Here, we determined the stellar mass accretion rate and its variability based on Transiting Exoplanet Survey Satellite and High-Accuracy Radial velocity Planetary Searcher (HARPS) observations. The stellar light curve shows a strong signal with a 3.03 0.06 days period, which we attribute to stellar rotation. Our analysis of the HARPS spectra shows a rotational velocity of , indicating that the inclination of the rotation axis is 50° 8°. This implies that the rotation axes of the star and its circumstellar disk are parallel within the measurement error. We apply magnetospheric accretion models to fit the profiles of the H line and derive mass accretion rates onto the star in the range of , varying over the rotation phase. The measured accretion rates are in agreement with those estimated from near-UV fluxes using accretion shock models. The derived accretion rates are higher than expected from the disk mass and planets' properties for the low values of the viscous parameter suggested by recent studies, potentially pointing to an additional mass reservoir in the inner disk to feed the accretion, such as a dead zone. We find that the He I λ10830 line shows a blueshifted absorption feature, indicative of a wind. The mass-loss rate estimated from the line depth is consistent with an accretion-driven inner disk MHD wind.
The Dynamic, Chimeric Inner Disk of PDS 70 Gaidos, Eric; Thanathibodee, Thanawuth; Hoffman, Andrew ...
The Astrophysical journal,
05/2024, Letnik:
966, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Abstract Transition disks, with inner regions depleted in dust and gas, could represent later stages of protoplanetary disk evolution when newly formed planets are emerging. The PDS 70 system has ...attracted particular interest because of the presence of two giant planets in orbits at tens of astronomical units within the inner disk cavity, at least one of which is itself accreting. However, the region around PDS 70 most relevant to understanding the planet populations revealed by exoplanet surveys of middle-aged stars is the inner disk, which is the dominant source of the system’s excess infrared emission but only marginally resolved by the Atacama Large Millimeter/submillimeter Array. Here we present and analyze time-series optical and infrared photometry and spectroscopy that reveal the inner disk to be dynamic on timescales of days to years, with occultation by submicron dust dimming the star at optical wavelengths, and 3–5 μ m emission varying due to changes in disk structure. Remarkably, the infrared emission from the innermost region (nearly) disappears for ∼1 yr. We model the spectral energy distribution of the system and its time variation with a flattened warm ( T ≲ 600 K) disk and a hotter (1200 K) dust that could represent an inner rim or wall. The high dust-to-gas ratio of the inner disk, relative to material accreting from the outer disk, means that the former could be a chimera consisting of depleted disk gas that is subsequently enriched with dust and volatiles produced by collisions and evaporation of planetesimals in the inner zone.
Abstract
We present a study of the Ca
ii
K and IR triplet lines in a sample of classical T Tauri stars in the Chamaeleon I star-forming region. We study X-shooter spectra of the stars in the sample ...and find that in some of these stars the Ca
ii
lines are much weaker than expected from their H line fluxes and mass accretion rates. Since the Ca
ii
K lines have characteristic magnetospheric accretion line profiles and the magnetospheric flows feed directly from the inner disk, we interpret the Ca deficiency in terms of depletion due to processes happening in the disk. To test this hypothesis, we define a coarse depletion indicator using the flux of the Ca
ii
K line and show that it correlates with disk properties. In particular, using indicators extracted from Spitzer/IRS spectra, we obtain that all the transitional and pretransitional disks of the sample show depletion, consistent with the trapping of refractories in pressure bumps created by planets and/or in the planets themselves. We find full disks with Ca depletion in the sample that also show indications of advanced dust evolution. We apply magnetospheric accretion models to fit the Balmer and Ca
ii
line fluxes of a star showing clear Ca depletion and derive a Ca abundance in its inner disk of about 17% solar.
Abstract
The detection of emission lines associated with accretion processes is a direct method for studying how and where gas giant planets form, how young planets interact with their natal ...protoplanetary disk, and how volatile delivery to their atmosphere takes place. H
α
(
λ
= 0.656
μ
m) is expected to be the strongest accretion line observable from the ground with adaptive optics systems, and is therefore the target of specific high-contrast imaging campaigns. We present MagAO-X and Hubble Space Telescope (HST) data obtained to search for H
α
emission from the previously detected protoplanet candidate orbiting AS209, identified through Atacama Large Millimeter/submillimeter Array observations. No signal was detected at the location of the candidate, and we provide limits on its accretion. Our data would have detected an H
α
emission with
F
H
α
> 2.5 ± 0.3 × 10
−16
erg s
−1
cm
−2
, a factor 6.5 lower than the HST flux measured for PDS70 b. The flux limit indicates that if the protoplanet is currently accreting it is likely that local extinction from circumstellar and circumplanetary material strongly attenuates its emission at optical wavelengths. In addition, the data reveal the first image of the jet north of the star as expected from previous detections of forbidden lines. Finally, this work demonstrates that current ground-based observations with extreme adaptive optics systems can be more sensitive than space-based observations, paving the way to the hunt for small planets in reflected light with extremely large telescopes.
On UT 29 June 2015, the occultation by Pluto of a bright star (r′ = 11.9) was observed from the Stratospheric Observatory for Infrared Astronomy (SOFIA) and several ground-based stations in New ...Zealand and Australia. Pre-event astrometry allowed for an in-flight update to the SOFIA team with the result that SOFIA was deep within the central flash zone (~22 km from center). Analysis of the combined data leads to the result that Pluto's middle atmosphere is essentially unchanged from 2011 and 2013 (Person et al. 2013; Bosh et al. 2015); there has been no significant expansion or contraction of the atmosphere. Additionally, our multi-wavelength observations allow us to conclude that a haze component in the atmosphere is required to reproduce the light curves obtained. This haze scenario has implications for understanding the photochemistry of Pluto's atmosphere.
Abstract
Advances in high-resolution imaging have revealed H
α
emission separate from the host star. It is generally believed that the emission is associated with planets forming in protoplanetary ...disks. However, the nature of this emission is still not fully understood. Here we report a modeling of H
α
emission from the planets around the young star PDS 70. Using standard magnetospheric accretion models previously applied to accreting young stars, we find that the observed line fluxes can be reproduced using a range of parameters relevant to PDS 70b and c, with a mean mass accretion rate of log(
) = −8.0 ± 0.6
M
Jup
yr
−1
and −8.1 ± 0.6
M
Jup
yr
−1
for PDS 70b and PDS 70c, respectively. Our results suggest that H
α
emission from young planets can originate in the magnetospheric accretion of mass from the circumplanetary disk. We find that empirical relationships between mass accretion rate and H
α
line properties frequently used in T Tauri stars are not applicable in the planetary mass regime. In particular, the correlations between line flux and mass accretion rate underpredict the accretion rate by about an order of magnitude, and the width at the 10% height of the line is insensitive to the accretion rate at
< 10
−8
M
Jup
yr
−1
.
Advances in high-resolution imaging have revealed H emission separate from the host star. It is generally believed that the emission is associated with planets forming in protoplanetary disks. ...However, the nature of this emission is still not fully understood. Here we report a modeling of H emission from the planets around the young star PDS 70. Using standard magnetospheric accretion models previously applied to accreting young stars, we find that the observed line fluxes can be reproduced using a range of parameters relevant to PDS 70b and c, with a mean mass accretion rate of log( ) = −8.0 0.6 MJup yr−1 and −8.1 0.6 MJup yr−1 for PDS 70b and PDS 70c, respectively. Our results suggest that H emission from young planets can originate in the magnetospheric accretion of mass from the circumplanetary disk. We find that empirical relationships between mass accretion rate and H line properties frequently used in T Tauri stars are not applicable in the planetary mass regime. In particular, the correlations between line flux and mass accretion rate underpredict the accretion rate by about an order of magnitude, and the width at the 10% height of the line is insensitive to the accretion rate at < 10−8 MJup yr−1.
A Census of the Low Accretors. I. The Catalog Thanathibodee, Thanawuth; Calvet, Nuria; Hernández, Jesús ...
The Astronomical journal,
02/2022, Letnik:
163, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Abstract
Observations have shown that the disk frequency and the fraction of accreting pre-main-sequence stars decrease with the age of the population and that some stars appear to have disks while ...their accretion has stopped. Still, it is unclear how disk-bearing stars stop their accretion. To provide insight into the last stages of accretion in low-mass young stars, we conducted a survey of disk-bearing stars that are thought to be non-accretors to identify stars still accreting at very low rates. Here we present the first catalog of the survey of 170 disk-bearing non-accreting stars in Chamaeleon I, Orion OB1, Upper Scorpius,
γ
Velorum, and Upper Centaurus–Lupus, using He
i
λ
10830 as a sensitive probe of accretion. We classify the line profiles into six types and argue that those showing redshifted and/or blueshifted absorption are still accreting. Using these classifications, we found that, among disk-bearing stars previously classified as non-accretors, at least 20%–30% are still accreting, with a larger fraction of those at younger population ages. While the difference between the outer disk signature and accretion status is unclear, we found a difference between the inner disk excess and accretion status. There is no preference in the mass of the newly identified accretors, suggesting that the processes inhibiting accretion do not directly depend on mass in the typical mass range of T Tauri stars. Lastly, we found that at a low accretion level, the “H
α
width at the 10% height” criterion mischaracterizes a larger fraction of accretors than the line’s equivalent width.
We report the detection of several absorption lines of neutral phosphorus (P, Z = 15) in archival near-ultraviolet spectra obtained with the Space Telescope Imaging Spectrograph on board the Hubble ...Space Telescope. We derive phosphoms abundances or interesting upper limits in 14 late-type stars with metallicities spanning -3.8 < Fe/H < -0.1. Previously, phosphorus had only been studied in Galactic stars with -1.0 < Fe/H < +0.3. Iron lines reveal abundance offsets between the optical and ultraviolet regions, and we discuss and apply a correction factor to account for this offset. In stars with Fe/H > -1.0, the P/Fe ratio decreases toward the solar value with increasing metallicity, in agreement with previous observational studies. In stars with Fe/H < -1.0, left angle bracketP/Feright angle bracket = +0.04 + or - 0.10, which overlaps with the P/Fe ratios found in several high-redshift damped Lyman- alpha systems. This behavior hints at a primary origin in massive stars.
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