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Register a new userHubble Space Telescope UV and H$alpha$ Measurements of the Accretion Excess Emission from the Young Giant Planet PDS 70 b
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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.
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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$^{-1}$.
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.
Recent observations have detected excess H$alpha$ emission from young stellar systems with an age of several Myr such as PDS 70. One-dimensional radiation-hydrodynamic models of shock-heated flows that we developed previously demonstrate that planetary accretion flows of $>$ a few ten km s$^{-1}$ can produce H$alpha$ emission. It is, however, a challenge to understand the accretion process of proto-giant planets from observations of such shock-originated emission because of a huge gap in scale between the circumplanetary disk (CPD) and the microscopic accretion shock. To overcome the scale gap problem, we combine two-dimensional, high-spatial-resolution global hydrodynamic simulations and the one-dimensional local radiation hydrodynamic model of the shock-heated flow. From such combined simulations for the protoplanet-CPD system, we find that the H$alpha$ emission is mainly produced in localized areas on the protoplanetary surface. The accretion shocks above CPD produce much weaker H$alpha$ emission (approximately 1-2 orders of magnitude smaller in luminosity). Nevertheless, the accretion shocks above CPD significantly affect the accretion process onto the protoplanet. The accretion occurs at a quasi-steady rate, if averaged on a 10-day timescale, but its rate shows variability on shorter timescales. The disk surface accretion layers including the CPD-shocks largely fluctuate, which results in the time-variable accretion rate and H$alpha$ luminosity of the protoplanet. We also model the spectral emission profile of the H$alpha$ line and find that the line profile is less time-variable, despite the large variability in luminosity. High-spectral resolution spectroscopic observation and monitoring will be key to reveal the property of the accretion process.
As host to two accreting planets, PDS 70 provides a unique opportunity to probe the chemical complexity of atmosphere-forming material. We present ALMA Band 6 observations of the PDS~70 disk and report the first chemical inventory of the system. With a spatial resolution of 0.4-0.5 ($sim$50 au), 12 species are detected, including CO isotopologues and formaldehyde, small hydrocarbons, HCN and HCO+ isotopologues, and S-bearing molecules. SO and CH3OH are not detected. All lines show a large cavity at the center of the disk, indicative of the deep gap carved by the massive planets. The radial profiles of the line emission are compared to the (sub-)mm continuum and infrared scattered light intensity profiles. Different molecular transitions peak at different radii, revealing the complex interplay between density, temperature and chemistry in setting molecular abundances. Column densities and optical depth profiles are derived for all detected molecules, and upper limits obtained for the non detections. Excitation temperature is obtained for H2CO. Deuteration and nitrogen fractionation profiles from the hydro-cyanide lines show radially increasing fractionation levels. Comparison of the disk chemical inventory to grids of chemical models from the literature strongly suggests a disk molecular layer hosting a carbon to oxygen ratio C/O>1, thus providing for the first time compelling evidence of planets actively accreting high C/O ratio gas at present time.
The recent high spatial/spectral resolution observations have enabled constraining formation mechanisms of giant planets, especially at the final stages. The current interpretation of such observations is that these planets undergo magnetospheric accretion, suggesting the importance of planetary magnetic fields. We explore the properties of accreting, magnetized giant planets surrounded by their circumplanetary disks, using the physical parameters inferred for PDS 70 b/c. We compute the magnetic field strength and the resulting spin rate of giant planets, and find that these planets may possess dipole magnetic fields of either a few 10 G or a few 100 G; the former is the natural outcome of planetary growth and radius evolution, while the resulting spin rate cannot reproduce the observations. For the latter, a consistent picture can be drawn, where strong magnetic fields induced by hot planetary interiors lead both to magnetospheric accretion and to spin-down due to disk locking. We also compute the properties of circumplanetary disks in the vicinity of these planets, taking into account planetary magnetic fields. The resulting surface density becomes very low, compared with the canonical models, implying the importance of radial movement of satellite-forming materials. Our model predicts a positive gradient of the surface density, which invokes the traps for both satellite migration and radially drifting dust particles. This work thus concludes that the final formation stages of giant planets are similar to those of low-mass stars such as brown dwarfs, as suggested by recent studies.
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