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Register a new userConstraining Planetary Gas Accretion Rate from H{alpha} Linewidth and Intensity: Case of PDS 70 b and c
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Recent observations of protoplanets embedded in circumstellar disks have shed light on the planet formation process. In particular, detection of hydrogen Balmer-line (H{alpha}) emission gives direct constraints on late-stage accretion onto gas giants. Very recently Haffert et al. (2019) measured the spectral line-widths, in addition to intensities, of H{alpha} emission from the two protoplanets orbiting PDS 70. Here, we study these protoplanets by applying radiation-hydrodynamic models of the shock-heated accretion flow onto protoplanets that Aoyama et al. (2018) has recently developed. As a result, we demonstrate that H{alpha} line-widths combined with intensities lead to narrowing down the possible ranges of the protoplanetary accretion rate and/or mass significantly. While the current spectral resolution is not high enough to derive a definite conclusion regarding their accretion process, high-resolution spectral imaging of growing protoplanets is highly promising.
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Recent discoveries of young exoplanets within their natal disks offer exciting opportunities to study ongoing planet formation. In particular, a planets mass accretion rate can be constrained by observing the accretion-induced excess emission. So far, planetary accretion is only probed by the H$alpha$ line, which is then converted to a total accretion luminosity using correlations derived for stars. However, the majority of the accretion luminosity is expected to emerge from hydrogen continuum emission, and is best measured in the ultraviolet (UV). In this paper, we present HST/WFC3/UVIS F336W (UV) and F656N (H$alpha$) high-contrast imaging observations of PDS 70. Applying a suite of novel observational techniques, we detect the planet PDS 70 b with signal-to-noise ratios of 5.3 and 7.8 in the F336W and F656N bands, respectively. This is the first time that an exoplanet has been directly imaged in the UV. Our observed H$alpha$ flux of PDS 70 b is higher by $3.5sigma$ than the most recent published result. However, the light curve retrieved from our observations does not support greater than 30% variability in the planets H$alpha$ emission in six epochs over a five-month timescale. We estimate a mass accretion rate of $1.4pm0.2times10^{-8}M_{mathrm{Jup}}/mathrm{yr}$. H$alpha$ accounts for 36% of the total accretion luminosity. Such a high proportion of energy released in line emission suggests efficient production of H$alpha$ emission in planetary accretion, and motivates using the H$alpha$ band for searches of accreting planets. These results demonstrate HST/WFC3/UVISs excellent high-contrast imaging performance and highlight its potential for planet formation studies.
Advances in high-resolution imaging have revealed H$alpha$ emission separated from the host star. It is generally believed that the emission is associated with forming planets in protoplanetary disks. However, the nature of this emission is still not fully understood. Here we report a modeling effort of H$alpha$ 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 the mean mass accretion rate of log(${rm dot{M}}$) = $-8.0pm0.6$ M$_{rm Jup}$ yr$^{-1}$ and $-8.1pm0.6$ M$_{rm Jup}$ yr$^{-1}$ for PDS 70b and PDS 70c, respectively. Our results suggest that H$alpha$ 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$alpha$ 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 ${rm dot{M}}$ $< 10^{-8}$ M$_{rm Jup}$ yr$^{-1}$.
We present K-band interferometric observations of the PDS 70 protoplanets along with their host star using VLTI/GRAVITY. We obtained K-band spectra and 100 $mu$as precision astrometry of both PDS 70 b and c in two epochs, as well as spatially resolving the hot inner disk around the star. Rejecting unstable orbits, we found a nonzero eccentricity for PDS 70 b of $0.17 pm 0.06$, a near-circular orbit for PDS 70 c, and an orbital configuration that is consistent with the planets migrating into a 2:1 mean motion resonance. Enforcing dynamical stability, we obtained a 95% upper limit on the mass of PDS 70 b of 10 $M_textrm{Jup}$, while the mass of PDS 70 c was unconstrained. The GRAVITY K-band spectra rules out pure blackbody models for the photospheres of both planets. Instead, the models with the most support from the data are planetary atmospheres that are dusty, but the nature of the dust is unclear. Any circumplanetary dust around these planets is not well constrained by the planets 1-5 $mu$m spectral energy distributions (SEDs) and requires longer wavelength data to probe with SED analysis. However with VLTI/GRAVITY, we made the first observations of a circumplanetary environment with sub-au spatial resolution, placing an upper limit of 0.3~au on the size of a bright disk around PDS 70 b.
We present the first observational evidence for a circumplanetary disk around the protoplanet PDS~70~b, based on a new spectrum in the $K$ band acquired with VLT/SINFONI. We tested three hypotheses to explain the spectrum: Atmospheric emission from the planet with either (1) a single value of extinction or (2) variable extinction, and (3) a combined atmospheric and circumplanetary disk model. Goodness-of-fit indicators favour the third option, suggesting circumplanetary material contributing excess thermal emission --- most prominent at $lambda gtrsim 2.3 mu$m. Inferred accretion rates ($sim 10^{-7.8}$--$10^{-7.3} M_J$ yr$^{-1}$) are compatible with observational constraints based on the H$alpha$ and Br$gamma$ lines. For the planet, we derive an effective temperature of 1500--1600 K, surface gravity $log(g)sim 4.0$, radius $sim 1.6 R_J$, mass $sim 10 M_J$, and possible thick clouds. Models with variable extinction lead to slightly worse fits. However, the amplitude ($Delta A_V gtrsim 3$mag) and timescale of variation ($lesssim$~years) required for the extinction would also suggest circumplanetary material.
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 TESS and HARPS observations. The stellar light curve shows a strong signal with a $3.03pm0.06$ days period, which we attribute to stellar rotation. Our analysis of the HARPS spectra shows a rotational velocity of $vsin,i=16.0pm0.5,{rm km,s^{-1}}$, indicating that the inclination of the rotation axis is $50pm8$ degrees. 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$alpha$ line and derive mass accretion rates onto the star in the range of $0.6-2.2times10^{-10},{rm M_{odot}yr^{-1}}$, varying over the rotation phase. The measured accretion rates are in agreement with those estimated from NUV 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 $alpha$ 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 $lambda$10830 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.
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