Estimates Of Mass Accretion Rate Research Articles

Near-infrared Accretion Signatures from the Circumbinary Planetary-mass Companion Delorme 1 (AB)b* * Based on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministério da Ciência, Tecnologia e Inovações (MCTI/LNA) do Brasil, the US National Science Foundation's NOIRLab, the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU).

Accretion signatures from bound brown dwarf and protoplanetary companions provide evidence for ongoing planet formation, and accreting substellar objects have enabled new avenues to study the astrophysical mechanisms controlling the formation and accretion processes. Delorme 1 (AB)b, a ∼30–45 Myr circumbinary planetary-mass companion, was recently discovered to exhibit strong Hα emission. This suggests ongoing accretion from a circumplanetary disk, somewhat surprising given canonical gas disk dispersal timescales of 5–10 Myr. Here, we present the first NIR detection of accretion from the companion in Paβ, Paγ, and Brγ emission lines from SOAR/TripleSpec 4.1, confirming and further informing its accreting nature. The companion shows strong line emission, with L line ≈ 1–6 × 10−8 L ⊙ across lines and epochs, while the binary host system shows no NIR hydrogen line emission (L line < 0.32–11 × 10−7 L ⊙). Observed NIR hydrogen line ratios are more consistent with a planetary accretion shock than with local line excitation models commonly used to interpret stellar magnetospheric accretion. Using planetary accretion shock models, we derive mass accretion rate estimates of –4 × 10−8 M J yr−1, somewhat higher than expected under the standard star formation paradigm. Delorme 1 (AB)b’s high accretion rate is perhaps more consistent with formation via disk fragmentation. Delorme 1 (AB)b is the first protoplanet candidate with clear (signal-to-noise ratio ∼5) NIR hydrogen line emission.

Improved Orbital Constraints and Hα Photometric Monitoring of the Directly Imaged Protoplanet Analog HD 142527 B

Companions embedded in the cavities of transitional circumstellar disks have been observed to exhibit excess luminosity at Hα, an indication that they are actively accreting. We report 5 yr (2013–2018) of monitoring of the position and Hα excess luminosity of the embedded, accreting low-mass stellar companion HD 142527 B from the MagAO/VisAO instrument. We use pyklip, a Python implementation of the Karhunen–Loeve Image Processing algorithm, to detect the companion. Using pyklip forward modeling, we constrain the relative astrometry to 1–2 mas precision and achieve sufficient photometric precision (±0.2 mag, 3% error) to detect changes in the Hα contrast of the companion over time. In order to accurately determine the relative astrometry of the companion, we conduct an astrometric calibration of the MagAO/VisAO camera against 20 yr of Keck/NIRC2 images of the Trapezium cluster. We demonstrate agreement of our VisAO astrometry with other published positions for HD 142527 B, and use orbitize! to generate a posterior distribution of orbits fit to the relative astrometry of HD 142527 B. Our data suggest that the companion is close to periastron passage, on an orbit significantly misaligned with respect to both the wide circumbinary disk and the recently observed inner disk encircling HD 142527 A. We translate observed Hα contrasts for HD 142527 B into mass accretion rate estimates on the order of 4–9 × 10−10 M ⊙ yr−1. Photometric variation in the Hα excess of the companion suggests that the accretion rate onto the companion is variable. This work represents a significant step toward observing accretion-driven variability onto protoplanets, such as PDS 70 b&c.

On the Mass Accretion Rates of Herbig Ae/Be Stars. Magnetospheric Accretion or Boundary Layer?

Understanding how young stars gain their masses through disk-to-star accretion is of paramount importance in astrophysics. It affects our knowledge about the early stellar evolution, the disk lifetime and dissipation processes, the way the planets form on the smallest scales, or the connection to macroscopic parameters characterizing star-forming regions on the largest ones, among others. In turn, mass accretion rate estimates depend on the accretion paradigm assumed. For low-mass T Tauri stars with strong magnetic fields there is consensus that magnetospheric accretion (MA) is the driving mechanism, but the transfer of mass in massive young stellar objects with weak or negligible magnetic fields probably occurs directly from the disk to the star through a hot boundary layer (BL). The intermediate-mass Herbig Ae/Be (HAeBe) stars bridge the gap between both previous regimes and are still optically visible during the pre-main sequence phase, thus constituting a unique opportunity to test a possible change of accretion mode from MA to BL. This review deals with our estimates of accretion rates in HAeBes, critically discussing the different accretion paradigms. It shows that although mounting evidence supports that MA may extend to late-type HAes but not to early-type HBes, there is not yet a consensus on the validity of this scenario versus the BL one. Based on MA and BL shock modeling, it is argued that the ultraviolet regime could significantly contribute in the future to discriminating between these competing accretion scenarios.

Complex Magnetospheric Accretion Flows in the Low Accretor CVSO 1335

Abstract Although the magnetospheric accretion model has been extensively applied to T Tauri Stars with typical mass accretion rates, the regime of very low accretion is still not fully explored. Here we report multi-epoch observations and modeling of CVSO 1335, a 5 Myr old star of solar mass that is accreting mass from the disk, as evidenced by redshifted absorption in the Hα profile, but with very uncertain estimates of mass accretion rate using traditional calibrators. We use the accretion shock model to constrain the mass accretion rate from the Balmer jump excess measured with respect to a non-accreting template, and we model the Hα profile, observed simultaneously, using magnetospheric accretion models. Using data taken on consecutive nights, we found that the accretion rate of the star is low, (4–9) × 10−10 M ⊙ yr−1, suggesting a variability on a timescale of days. The observed Hα profiles point to two geometrically isolated accretion flows, suggesting a complex infall geometry. The systems of redshifted absorptions observed are consistent with the star being a dipper, although multi-band photometric monitoring is needed to confirm this hypothesis.

PRE-MAIN-SEQUENCE ACCRETION IN THE LOW METALLICITY GALACTIC STAR-FORMING REGION Sh 2-284

We present optical spectra of pre-main sequence (PMS) candidates around the H$\alpha$ region taken with the Southern African Large Telescope, SALT, in the low metallicity ($Z$) Galactic region Sh 2-284, which includes the open cluster Dolidze 25 with an atypical low metallicity of $Z$ $\sim$ 1/5 $Z_{\odot}$. It has been suggested on the basis of both theory and observations that PMS mass-accretion rates, $\dot M_{\rm{acc}}$, are a function of $Z$. We present the first sample of spectroscopic estimates of mass-accretion rates for PMS stars in any low-$Z$ star-forming region. Our data-set was enlarged with literature data of H$\alpha$ emission in intermediate-resolution R-band spectroscopy. Our total sample includes 24 objects spanning a mass range between 1 - 2 $M_{\odot}$ and with a median age of approximately 3.5 Myr. The vast majority (21 out of 24) show evidence for a circumstellar disk on the basis of 2MASS and Spitzer infrared photometry. We find $\dot M_{\rm{acc}}$ in the 1 - 2 $M_{\odot}$ interval to depend quasi-quadratically on stellar mass, with $\dot M_{\rm{acc}}$ $\propto$ $M_{\ast}^{2.4\,\pm\,0.35}$, and inversely with stellar age $\dot M_{\rm{acc}}$ $\propto$ $t_{\ast}^{-\,0.7\,\pm\,0.4}$. Furthermore, we compare our spectroscopic $\dot M_{\rm{acc}}$ measurements with solar $Z$ Galactic PMS stars in the same mass range, but, surprisingly find no evidence for a systematic change in $\dot M_{\rm{acc}}$ with $Z$. We show that literature accretion-rate studies are influenced by detection limits, and we suggest that $\dot M_{\rm{acc}}$ may be controlled by factors other than $Z_{\ast}$, $M_{\ast}$, and age.

X-shooter spectroscopy of young stellar objects

Context. The lack of knowledge of photospheric parameters and the level of chromospheric activity in young low-mass pre-main sequence stars introduces uncertainties when measuring mass accretion rates in accreting (Class II) young stellar objects. A detailed investigation of the effect of chromospheric emission on the estimates of mass accretion rate in young low-mass stars is still missing. This can be undertaken using samples of young diskless (Class III) K and M-type stars.

Microlensing evidence for super-Eddington disc accretion in quasars

Microlensing by the stellar population of lensing galaxies provides an important opportunity to spatially resolve the accretion disc structure in strongly lensed quasars. Disc sizes estimated this way are on average larger than the predictions of the standard Shakura-Sunyaev accretion disk model. Analysing the observational data on microlensing variability allows to suggest that some fraction of lensed quasars (primarily, smaller-mass objects) are accreting in super-Eddington regime. Super-Eddington accretion leads to formation of an optically-thick envelope scattering the radiation formed in the disc. This makes the apparent disc size larger and practically independent of wavelength. In the framework of our model, it is possible to make self-consistent estimates of mass accretion rates and black hole masses for the cases when both amplification-corrected fluxes and radii are available.

Stellar parameters of young brown dwarfs

AbstractWe present X‐shooter observations of two brown dwarf candidates. We focus on the determination of stellar parameters and their errors. The targets, an accreting class II and a non‐accreting class III objects, are members of the σ Orionis star‐forming region. We derive the spectroscopic spectral types from the VIS spectrum and the stellar parameters. We find that the uncertainties on the stellar parameters have a minor effect on the determination of the mass accretion rate for the accreting star, thus confirming that the discrepancies between the mass accretion rate estimates found with different (simultaneous) tracers are probably due to different physical conditions where the accretion/wind indicators are produced (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Star and protoplanetary disk properties in Orion's suburbs

(Note: this is a shortened version of the original "structured" A&A format abstract.) We performed a large optical spectroscopic and photometric survey of the Lynds~1630N and 1641 clouds. We provide a catalog of 132 confirmed young stars in L1630N and 267 such objects in L1641. We identify 28 transition disk systems, 20 of which were previously unknown, as well as 42 new transition disk candidates for which we have broad-band photometry but no optical spectroscopy. We estimate mass accretion rates M_acc from the equivalent widths of the H_alpha, H_beta, and HeI 5876\AA emission lines, and find a dependence on stellar mass of M_acc propto Mstar^alpha, with alpha~3.1 in the subsolar mass range that we probe. An investigation of a large literature sample of mass accretion rate estimates yields a similar slope of alpha~2.8 in the subsolar regime, but a shallower slope of alpha~2.0 if the whole mass range of 0.04 M_sun-5 Msun is included. Among the transition disk objects, the fraction of stars that show significant accretion activity is relatively low compared to stars with still optically thick disks (26\pm11% vs. 57\pm6%, respectively). However, those transition disks that do show significant accretion have the same median accretion rate as normal optically thick disks of 3-4*10^{-9} M_sun/yr. We find that the ages of the transition disks and the WTTSs without disks are statistically indistinguishable, and both groups are significantly older than the CTTSs. These results argue against disk-binary interaction or gravitational instability as mechanisms causing a transition disk appearance. Our observations indicate that disk lifetimes in the clustered population are shorter than in the distributed population. We propose refined Halpha equivalent width criteria to distinguish WTTSs from CTTSs.

Measuring Accretion in Young Substellar Objects: Approaching the Planetary Mass Regime

We present observations of Halpha emission line profiles taken at Magellan Observatory for a sample of 39 young low-mass stars and brown dwarfs in the Taurus and Chamaeleon I star forming regions. We have identified 11 new substellar accretors, more than tripling the number of known brown dwarfs with measurable accretion activity. These include the lowest-mass objects yet seen with accretion, with masses down to ~0.015 Msun. Using models of Halpha emission produced in magnetospheric accretion flows, the most widely applicable primary calibrator now available, we determine the first estimates of mass accretion rates for objects at such extremely low masses. For the six objects with masses < 0.03 Msun, we find accretion rates of ~ 5x10^{-12} Msun/yr, among the smallest yet measured. These new results continue the trend of decreasing mass accretion rate with decreasing (sub)stellar mass that we have noted previously for samples of more massive objects; the overall correlation is Mdot \~ M^{2.1}, and now extends over a mass range of over two orders of magnitude. Finally, the absence of a discontinuity in the distribution of accretion rates with mass tends to suggest that stars and brown dwarfs share similar formation histories.

The Observational Evidence for Accretion

Outflows from low-mass young stellar objects are thought to draw upon the energy released by accretion onto T Tauri stars. I briefly summarize the evidence for this accretion and outline present estimates of mass accretion rates. Young stars show a very large range of accretion rates, and this has important implications for both mass ejection and for the structure of stellar magnetospheres which may truncate T Tauri disks.