Accretion through circumstellar disks plays an important role in star formation and in establishing the properties of the regions in which planets form and migrate. The mechanisms by which protostellar and protoplanetary disks accrete onto low-mass stars are not clear; angular momentum transport by magnetic fields is thought to be involved, but the low-ionization conditions in major regions of protoplanetary disks lead to a variety of complex nonideal magnetohydrodynamic effects whose implications are not fully understood. Accretion in pre-main-sequence stars of masses ≲1M⊙ (and in at least some 2–3-M⊙ systems) is generally funneled by the stellar magnetic field, which disrupts the disk at scales typically of order a few stellar radii. Matter moving at near free-fall velocities shocks at the stellar surface; the resulting accretion luminosities from the dissipation of kinetic energy indicate that mass addition during the T Tauri phase over the typical disk lifetime ∼3 Myr is modest in terms of stellar evo...

Accretion onto Pre-Main-Sequence Stars

The origin of a recently discovered close-in Neptune-mass planet around GJ 436 poses a challenge to the current theories of planet formation. On the basis of the sequential accretion hypothesis and the standard theory of gap formation and orbital migration, we show that around M dwarf stars, close-in Neptune-mass ice giant planets may be relatively common, while close-in Jupiter-mass gas giant planets are relatively rare. The mass distribution of close-in planets generally has two peaks at about Neptune mass and Jupiter mass. The lower mass peak takes the maximum frequency for M dwarfs. Around more massive solar-type stars (G dwarfs), the higher mass peak is much more pronounced. Planets around G dwarfs undergo orbital migration after fully accreting gas, while those around M dwarfs tend to migrate before starting rapid gas accretion. Close-in Neptune-mass planets may also exist around G dwarfs, although they tend to be mostly composed of silicates and iron cores and their frequency is expected to be much smaller than that of Neptune-mass planets around M dwarfs and that of gas giants around G dwarfs. We also show that the conditions for planets' migration due to their tidal interaction with the disk and the stellar mass dependence in the disk mass distribution can be calibrated by the mass distribution of short-period planets around host stars with various masses.

pdf/toward-a-deterministic-model-of-planetary-formation-iii-mass-5gs3oc4fd9.pdf

Toward a Deterministic Model of Planetary Formation. III. Mass Distribution of Short-Period Planets around Stars of Various Masses

We present the initial result of a large spectroscopic survey aimed at measuring the timescale of mass accretion in young, pre-main-sequence stars in the spectral type range K0-M5. Using multi-object spectroscopy with VIMOS at the VLT we identified the fraction of accreting stars in a number of young stellar clusters and associations of the ages of between 1-30 Myr. The fraction of accreting stars decreases from ~60% at 1.5-2 Myr to ~2% at 10 Myr. No accreting stars are found after 10 Myr at a sensitivity limit of 10 -11 M ⊙ yr -1 We compared the fraction of stars showing ongoing accretion (f acc ) to the fraction of stars with near-to-mid infrared excess (f IRAC ). In most cases we find f acc < F IRAC , i.e., mass accretion appears to cease (or drop below detectable level) earlier than the dust is dissipated in the inner disk. At 5 Myr, 95% of the stellar population has stopped accreting material at a rate of ≳10 -11 M ⊙ yr -1 while ~20% of the stars show near-infrared excess emission. Assuming an exponential decay, we measure a mass accretion timescale (τ acc ) of 2.3 Myr, compared to a near-to-mid infrared excess timescale (τ IRAC ) of 3 Myr. Planet formation and/or migration, in the inner disk might be a viable mechanism to halt further accretion onto the central star on such a short timescale.

/pdf/timescale-of-mass-accretion-in-pre-main-sequence-stars-o2fizlbvjq.pdf

Timescale of mass accretion in pre-main-sequence stars

Newly forming protoplanets are expected to create cavities and substructures in young, gas-rich protoplanetary disks1–3, but they are difficult to detect as they could be confused with disk features affected by advanced image analysis techniques4,5. Recently, a planet was discovered inside the gap of the transitional disk of the T Tauri star PDS 706,7. Here, we report on the detection of strong Hα emission from two distinct locations in the PDS 70 system, one corresponding to the previously discovered planet PDS 70 b, which confirms the earlier Hα detection8, and another located close to the outer edge of the gap, coinciding with a previously identified bright dust spot in the disk and with a small opening in a ring of molecular emission6,7,9. We identify this second Hα peak as a second protoplanet in the PDS 70 system. The Hα emission spectra of both protoplanets indicate ongoing accretion onto the protoplanets10,11, which appear to be near a 2:1 mean motion resonance. Our observations show that adaptive-optics-assisted, medium-resolution integral field spectroscopy with MUSE12 targeting accretion signatures will be a powerful way to trace ongoing planet formation in transitional disks at different stages of their evolution. Finding more young planetary systems in mean motion resonance would give credibility to the Grand Tack hypothesis in which Jupiter and Saturn migrated in a resonance orbit during the early formation period of our Solar System13. Two Hα emission peaks are detected within the disk of the T Tauri star PDS 70: one corresponds to protoplanet PDS 70 b, and the other is associated with a second accreting planet of few Jupiter masses at ~35 au. The two protoplanets are near 2:1 mean motion resonance, supporting migration scenarios of giant planets during planetary formation.

Two accreting protoplanets around the young star PDS 70

We present VLT/X-shooter observations of a sample of 36 accreting low-mass stellar and substellar objects (YSOs) in the Lupus star-forming region, spanning a range in mass from ~0.03 to ~1.2 M ⊙ , but mostly with 0.1 M ⊙ ⋆ ⊙ . Our aim is twofold: firstly, to analyse the relationship between excess-continuum and line emission accretion diagnostics, and, secondly, to investigate the accretion properties in terms of the physical properties of the central object. The accretion luminosity (L acc ), and in turn the accretion rate (Ṁ acc ), was derived by modelling the excess emission from the UV to the near-infrared as the continuum emission of a slab of hydrogen. We computed the flux and luminosity (L line ) of many emission lines of H  , He  , and Ca   ii, observed simultaneously in the range from ~330 nm to 2500 nm. The luminosity of all the lines is well correlated with L acc . We provide empirical relationships between L acc and the luminosity of 39 emission lines, which have a lower dispersion than relationships previously reported in the literature. Our measurements extend the Paβ and Brγ relationships to L acc values about two orders of magnitude lower than those reported in previous studies. We confirm that different methodologies of measuring L acc and Ṁ acc yield significantly different results: Hα line profile modelling may underestimate Ṁ acc by 0.6 to 0.8 dex with respect to Ṁ acc derived from continuum-excess measures. These differences may explain the probably spurious bi-modal relationships between Ṁ acc and other YSOs properties reported in the literature. We derived Ṁ acc in the range 2 × 10-12 –4 × 10-8  M ⊙ yr-1 and conclude that Ṁ acc ∝ M ⋆ 1.8(±0.2) , with a dispersion lower by a factor of about 2 than in previous studies. A number of properties indicate that the physical conditions of the accreting gas are similar over more than 5 orders of magnitude in Ṁ acc , confirming previous suggestions that the geometry of the accretion flow controls the rate at which the disc material accretes onto the central star.

https://www.aanda.org/articles/aa/pdf/2014/01/aa22254-13.pdf

X-shooter spectroscopy of young stellar objects. IV. Accretion in low-mass stars and substellar objects in Lupus

This paper presents a study of the accretion properties of 19 very low mass objects (M*∼0.01-0.1M ○. ) in the regions Chamaeleon I and p Oph. For 8 objects we obtained high resolution Ha profiles and determined mass accretion rate M ac and accretion luminosity L ac . Paβ is detected in emission in 7 of the 10 p Oph objects, but only in one in Cha I. Using objects for which we have both a determination of L ac from Ha and a Paβ detection, we show that the correlation between the Paβ luminosity and luminosity L dc , found by Muzerolle et al. (1998) for T Tauri stars in Taurus, extends to objects with mass ∼0.03 M ○. ; L(Paβ) can be used to measure L ac also in the substellar regime. The results were less conclusive for Bry, which was detected only in 2 objects, neither of which had an He estimate of M ac . Using the relation between L(Paβ) and L ac we determined the accretion rate for all the objects in our sample (including those with no Ha spectrum), more than doubling the number of substellar objects with known M ac . When plotted as a function of the mass of the central object together with data from the literature, our results confirm the trend of lower M ac for lower M * , although with a large spread. Some of the spread is probably due to an age effect; our very young objects in p Oph have on average an accretion rate at least one order of magnitude higher than objects of similar mass in older regions. As a side product, we found that the width of Ha measured at 10% peak intensity is not only a qualitative indicator of the accreting nature of very low mass objects, but can be used to obtain a quantitative, although not very accurate, estimate of M ac over a large mass range, from T Tauri stars to brown dwarfs. Finally, we found that some of our objects show evidence of mass-loss in their optical spectra.

/pdf/accretion-in-brown-dwarfs-an-infrared-view-3hlwdg87rz.pdf

Accretion in brown dwarfs: An infrared view

This paper presents a study of the accretion properties of 19 very low mass objects (Mstar .01-0.1 Msun) in the regions Chamaeleon I and rho-Oph For 8 objects we obtained high resolution Halpha profiles and determined mass accretion rate Macc and accretion luminosity Lacc. Pabeta is detected in emission in 7 of the 10 rho-Oph objects, but only in one in Cha I. Using objects for which we have both a determination of Lacc from Halpha and a Pabeta detection, we show that the correlation between the Pabeta luminosity and luminosity Lacc, found by Muzerolle et al. (1998) for T Tauri stars in Taurus, extends to objects with mass approx 0.03 Msun; L(Pab) can be used to measure Lacc also in the substellar regime. The results were less conclusive for Brgamma, which was detected only in 2 objects, neither of which had an Halpha estimate of Macc. Using the relation between L(Pab) and Lacc we determined the accretion rate for all the objects in our sample (including those with no Halpha spectrum), more than doubling the number of substellar objects with known Macc. When plotted as a function of the mass of the central object together with data from the literature, our results confirm the trend of lower Macc for lower Mstar, although with a large spread. Some of the spread is probably due to an age effect; our very young objects in rho-Oph have on average an accretion rate at least one order of magnitude higher than objects of similar mass in older regions. As a side product, we found that the width of Halpha measured at 10% peak intensity is not only a qualitative indicator of accretion, but can be used to obtain a quantitative estimate of Macc over a large mass range, from T Tauri stars to brown dwarfs. Finally, we found that some of our objects show evidence of mass-loss.

/pdf/accretion-in-brown-dwarfs-an-infrared-view-1g6fa6hd0n.pdf

Accretion in Brown Dwarfs: an Infrared View

We present optical spectra of eight candidate brown dwarfs and a previously known T Tauri star (Sz 33) of the Chamaeleon I dark cloud. We derived spectral types based on the strength of the TiO or VO absorption bands present in the spectra of these objects as well as on the PC3 index of Mart´in et al. (1999). Photometric data from the literature are used to estimate the bolometric luminosities for these sources. We apply D'Antona & Mazzitelli (1997) pre-main sequence evolutionary tracks and isochrones to derive masses and ages. Based on the presence of H in emission, we confirm that most of the candidates are young objects. Our sample however includes two sources for which we can only provide upper limits for the emission in H; whereas these two objects are most likely foreground/background stars, higher resolution spectra are required to confirm their true nature. Among the likely cloud members, we detect one new sub-stellar object and three transition stellar/sub-stellar sources.

/pdf/optical-spectra-of-selected-chamaeleon-i-young-stellar-51ll1jtrtp.pdf

Optical spectra of selected Chamaeleon I young stellar objects

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Accretion in brown dwarfs: An infrared view

Accretion in Brown Dwarfs: an Infrared View

Optical spectra of selected Chamaeleon I young stellar objects