arXiv subjects

Yuhiko Aoyama

Publications and source records attributed to Yuhiko Aoyama.

31 records · Page 2Linked to original sources

arXiv2023

Planetary line-to-accretion luminosity scaling relations: Extrapolating to higher-order hydrogen lines

Aoyama et al. (2021, ApJL) provided scaling relations between hydrogen-line luminosities and the accretion luminosity for planetary-mass objects. These fits should be an improvement over blind extrapolations of stellar relations. The fits go up only to the n = 8 electron energy level, but higher-n Balmer lines have been observed in the near-UV at Delorme 1 (AB)b with UVES (Ringqvist et al. 2023). We extend the scaling relations to higher-n levels for the Balmer and other series by fitting the fit coefficients (a, b) themselves and extrapolating them. Within the assumption of an accretion shock as the source of line emission, these fits should be robust for accreting planetary-mass objects.

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arXiv2023

Three-dimensional Global Simulations of Type-II Planet-disk Interaction with a Magnetized Disk Wind: I. Magnetic Flux Concentration and Gap Properties

Giant planets embedded in protoplanetary disks (PPDs) can create annulus density gaps around their orbits in the type-II regime, potentially responsible for the ubiquity of annular substructures observed in PPDs. Despite of substantial amount of works studying type-II planet migration and gap properties, they are almost exclusively conducted under the viscous accretion disk framework. However, recent studies have established magnetized disk winds as the primary driving disk accretion and evolution, which can co-exist with turbulence from the magneto-rotational instability (MRI) in the outer PPDs. We conduct a series of 3D global non-ideal magneto-hydrodynamic (MHD) simulations of type-II planet-disk interaction applicable to the outer PPDs. Our simulations properly resolve the MRI turbulence and accommodate the MHD disk wind. We found that the planet triggers the poloidal magnetic flux concentration around its orbit. The concentrated magnetic flux strongly enhances angular momentum removal in the gap, which is along the inclined poloidal field through a strong outflow emanating from the disk surface outward of the planet gap. The resulting planet-induced gap shape is more similar to an inviscid disk, while being much deeper, which can be understood from a simple inhomogeneous wind torque prescription. The corotation region is characterized by a fast trans-sonic accretion flow that is asymmetric in azimuth about the planet and lacking the horseshoe turns, and the meridional flow is weakened. The torque acting on the planet generally drives inward migration, though the migration rate can be affected by the presence of neighboring gaps through stochastic, planet-free magnetic flux concentration.

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arXiv2023

Forbidden emission lines in protostellar outflows and jets with MUSE

Forbidden emission lines in protoplanetary disks are a key diagnostic in studies of the evolution of the disk and the host star. We report spatially resolved emission lines, [OI] 6300, 6363, [NII] 6548, 6583, H$α$, and [SII] 6716, 6730 Angstrom that are believed to be associated with jets and magnetically driven winds in the inner disks. Observations were carried out with the optical integral field spectrograph of the Multi Unit Spectroscopic Explorer (MUSE), at the Very Large Telescope (VLT). With a resolution of 0.025 X 0.025 arcsec$^{2}$, we aim to derive the position angle of the outflow/jet (PA$_{outflow/jet}$) that is connected with the inner disk. The forbidden emission lines analyzed here have their origin at the inner parts of the protoplanetary disk. From the maximum intensity emission along the outflow/jet in DL Tau, CI Tau, DS Tau, IP Tau, and IM Lup, we were able to reliably measure the PA$_{outflow/jet}$ for most of the identified lines. We found that our estimates agree with PA$_{dust}$ for most of the disks. These estimates depend on the signal-to-noise level and the collimation of the outflow (jet). The outflows/jets in CIDA 9, GO Tau, and GW Lup are too compact for a PA$_{outflow/jet}$ to be estimated. Based on our kinematics analysis, we confirm that DL Tau and CI Tau host a strong outflow/jet with line-of-sight velocities much greater than 100 km s$^{-1}$, whereas DS Tau, IP Tau, and IM Lup velocities are lower and their structures encompass low-velocity components to be more associated with winds. Our estimates for the mass-loss rate, $\dot{M}_{loss}$, range between (1.1-6.5)X10$^{-7}$-10$^{-8}$ $M_{\odot}$ yr$^{-1}$ for the disk-outflow/jet systems analyzed here. The outflow/jet systems analyzed here are aligned within around 1 degree between the inner and outer disk. Further observations are needed to confirm a potential misalignment in IM Lup.

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arXiv2021

Hydrodynamic Model of H$α$ Emission from Accretion Shocks of Proto-Giant Planet and Circumplanetary Disk

Recent observations have detected excess H$α$ 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$α$ 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$α$ emission is mainly produced in localized areas on the protoplanetary surface. The accretion shocks above CPD produce much weaker H$α$ 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$α$ luminosity of the protoplanet. We also model the spectral emission profile of the H$α$ 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.

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arXiv2021

Characterizing the protolunar disk of the accreting companion GQ Lupi B

GQ Lup B is a young and accreting, substellar companion that appears to drive a spiral arm in the circumstellar disk of its host star. We report high-contrast imaging observations of GQ Lup B with VLT/NACO at 4-5 $μ$m and medium-resolution integral field spectroscopy with VLT/MUSE. The optical spectrum is consistent with an M9 spectral type, shows characteristics of a low-gravity atmosphere, and exhibits strong H$α$ emission. The $H-M'$ color is $\gtrsim$1 mag redder than field dwarfs with similar spectral types and a detailed analysis of the spectral energy distribution (SED) from optical to mid-infrared wavelengths reveals excess emission in the $L'$, NB4.05, and $M'$ bands. The excess flux is well described by a blackbody component with $T_\mathrm{disk} \approx 460$ K and $R_\mathrm{disk} \approx 65\,R_\mathrm{J}$ and is expected to trace continuum emission from small grains in a protolunar disk. We derive an extinction of $A_V \approx 2.3$ mag from the broadband SED with a suspected origin in the vicinity of the companion. We also combine 15 yr of astrometric measurements and constrain the mutual inclination with the circumstellar disk to $84 \pm 9$ deg, indicating a tumultuous dynamical evolution or a stellar-like formation pathway. From the measured H$α$ flux and the estimated companion mass, $M_\mathrm{p} \approx 30\,M_\mathrm{J}$, we derive an accretion rate of $\dot{M} \approx 10^{-6.5}\,M_\mathrm{J}\,\mathrm{yr}^{-1}$. We speculate that the disk is in a transitional stage in which the assembly of satellites from a pebble reservoir has opened a central cavity while GQ Lup B is in the final stages of its formation.

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arXiv2021

Keck/OSIRIS Pa$β$ high-contrast imaging and updated constraints on PDS~70b

We present a high-contrast imaging search for Pa$β$ line emission from protoplanets in the PDS~70 system with Keck/OSIRIS integral field spectroscopy. We applied the high-resolution spectral differential imaging technique to the OSIRIS $J$-band data but did not detect the Pa$β$ line at the level predicted using the parameters of \cite{Hashimoto2020}. This lack of Pa$β$ emission suggests the MUSE-based study may have overestimated the line width of H$α$. We compared our Pa$β$ detection limits with the previous H$α$ flux and H$β$ limits and estimated $A_{\rm V}$ to be $\sim0.9$ and 2.0 for PDS~70~b and c respectively. In particular, PDS~70~b's $A_{\rm V}$ is much smaller than implied by high-contrast near-infrared studies, which suggests the infrared-continuum photosphere and the hydrogen-emitting regions exist at different heights above the forming planet.

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arXiv2021

Comparison of planetary Hα-emission models: A new correlation with accretion luminosity

Accreting planets have been detected through their hydrogen-line emission, specifically H$α$. To interpret this, stellar-regime empirical correlations between the H$α$ luminosity $L_\mathrm{Hα}$ and the accretion luminosity $L_\mathrm{acc}$ or accretion rate $\dot{M}$ have been extrapolated to planetary masses, however without validation. We present a theoretical $L_\mathrm{acc}$--$L_\mathrm{Hα}$ relationship applicable to a shock at the surface of a planet. We consider wide ranges of accretion rates and masses and use detailed spectrally-resolved, non-equilibrium models of the postshock cooling. The new relationship gives a markedly higher $L_\mathrm{acc}$ for a given $L_\mathrm{Hα}$ than fits to young stellar objects, because Ly-$α$, which is not observable, carries a large fraction of $L_\mathrm{acc}$. Specifically, an $L_\mathrm{Hα}$ measurement needs ten to 100 times higher $L_\mathrm{acc}$ and $\dot{M}$ than previously predicted, which may explain the rarity of planetary H$α$ detections. We also compare the $\dot{M}$--$L_\mathrm{Hα}$ relationships coming from the planet-surface shock or implied by accretion-funnel emission. Both can contribute simultaneously to an observed H$α$ signal but at low (high) $\dot{M}$ the planetary-surface shock (heated funnel) dominates. Only the shock produces Gaussian line wings. Finally, we discuss accretion contexts in which different emission scenarios may apply, putting recent literature models in perspective, and also present $L_\mathrm{acc}$--$L_\mathrm{line}$ relationships for several other hydrogen lines.

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arXiv2021

Spectral appearance of the planetary-surface accretion shock: Global spectra and hydrogen-line profiles and fluxes

Hydrogen-line emission from an accretion shock has recently been observed at planetary-mass objects. Our previous work predicted the shock spectrum and luminosity for a shock on the circumplanetary disc. We extend this to the planet-surface shock. We calculate the global spectral energy distribution (SED) of accreting planets by combining our model emission spectra with photospheric SEDs, and predict the line-integrated flux for several hydrogen lines, especially H alpha, but also H beta, Pa alpha, Pa beta, Pa gamma, Br alpha, and Br gamma. We apply our non-equilibrium emission model to the surface accretion shock for a wide range of accretion rates Mdot and masses M_p . Fits to formation calculations provide radii and effective temperatures. Extinction by the surrounding material is neglected, which is arguably often relevant. We find that the line luminosity increases monotonically with Mdot and M_p , depending mostly on Mdot and weakly on M_p for the relevant range of parameters. The Lyman, Balmer, and Paschen continua can exceed the photosphere. The H beta line is fainter by 0 to 1 dex than H alpha, whereas other lines are weaker (by 1 to 3 dex). Shocks on the planet or the CPD surface are distinguishable at very high spectral resolution, but the planet surface shock likely dominates if both are present. Applied to recent non-detections of H alpha, our models imply looser constraints on the Mdot of putative planets than when extrapolating fits from the stellar regime. These hydrogen-line luminosity predictions are useful for interpreting (non-)detections of accreting planets.

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arXiv2020

Strong H$α$ emission and signs of accretion in a circumbinary planetary mass companion from MUSE

Context. Intrinsic H$α$ emission can be advantageously used to detect substellar companions because it improves contrasts in direct imaging. Characterising this emission from accreting exoplanets allows for the testing of planet formation theories. Aims. We characterise the young circumbinary planetary mass companion 2MASS J01033563-5515561 (AB)b (Delorme 1 (AB)b) through medium-resolution spectroscopy. Methods. We used the new narrow-field mode (NFM) for the MUSE integral-field spectrograph, located on the ESO Very Large Telescope, during science verification time to obtain optical spectra of Delorme 1 (AB)b. Results. We report the discovery of very strong H$α$ and H$β$ emission, accompanied by He I emission. This is consistent with an active accretion scenario. We provide accretion rate estimates obtained from several independent methods and find a likely mass of $12-15$ M$_{\rm Jup}$ for Delorme 1 (AB)b. This is also consistent with previous estimates. Conclusions. Signs of active accretion in the Delorme 1 system might indicate a younger age than the $\sim 30-40$ Myr expected from a likely membership in Tucana-Horologium (THA). Previous works have also shown the central binary to be overluminous, which gives further indication of a younger age. However, recent discoveries of active discs in relatively old ($\sim 40$ Myr), very low-mass systems suggests that ongoing accretion in Delorme 1 (AB)b might not require in and of itself that the system is younger than the age implied by its THA membership.

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arXiv2020

Accretion Properties of PDS 70b with MUSE

We report a new evaluation of the accretion properties of PDS~70b obtained with VLT/MUSE. The main difference from previous studies in Haffert et al. (2019) and Aoyama & Ikoma (2019) is in the mass accretion rate. Simultaneous multiple line observations, such as H$α$ and H$β$, can better constrain the physical properties of an accreting planet. While we clearly detected H$α$ emissions from PDS~70b, no H$β$ emissions were detected. We estimate the line flux of H$β$ with a 3-$σ$ upper limit to be 2.3~$\times$~10$^{-16}$~erg~s$^{-1}$~cm$^{-2}$. The flux ratio $F_{\rm Hβ}$/$F_{\rm Hα}$ for PDS~70b is $<$~0.28. Numerical investigations by Aoyama et al. (2018) suggest that $F_{\rm Hβ}$/$F_{\rm Hα}$ should be close to unity if the extinction is negligible. We attribute the reduction of the flux ratio to the extinction, and estimate the extinction of H$α$ ($A_{\rm Hα}$) for PDS~70b to be $>$~2.0~mag using the interstellar extinction value. %The expected $A_{\rm Hα}$ value in the gap of the protoplanetary disk at the PDS~70b location is 2.4~mag, which is consistent with the estimated extinction. By combining with the H$α$ linewidth and the dereddening line luminosity of H$α$, %we derive the PDS~70b dynamical mass and mass accretion rate to be \hashimotor{12~$\pm$~3~$M_{\rm Jup}$} and $\gtrsim$~5~$\times$~10$^{-7}$~$M_{\rm Jup}$~yr$^{-1}$, respectively. we derive the PDS~70b mass accretion rate to be $\gtrsim$~5~$\times$~10$^{-7}$~$M_{\rm Jup}$~yr$^{-1}$. The PDS~70b mass accretion rate is an order of magnitude larger than that of PDS~70. We found that the filling factor $f_{\rm f}$ (the fractional area of the planetary surface emitting H$α$) is $\gtrsim$0.01, which is similar to the typical stellar value. The small value of $f_{\rm f}$ indicates that the H$α$ emitting areas are localized at the surface of PDS~70b.

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arXiv2019

Constraining Planetary Gas Accretion Rate from Hα Linewidth and Intensity: Case of PDS 70 b and c

Recent observations of protoplanets embedded in circumstellar disks have shed light on the planet formation process. In particular, detection of hydrogen Balmer-line (Hα) 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 Hα 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 Hα 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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arXiv2018

Theoretical Model of Hydrogen Line Emission from Accreting Gas Giants

Progress in understanding of giant planet formation has been hampered by a lack of observational constraints to growing protoplanets. Recently, detection of an Hα-emission excess via direct imaging was reported for the protoplanet LkCa15b orbiting the pre-main-sequence star LkCa15. However, the physical mechanism for the Hα emission is poorly understood. According to recent high-resolution three-dimensional hydrodynamic simulations of the flow accreting onto protoplanets, the disk gas flows down almost vertically onto and collides with the surface of a circum-planetary disk at a super-sonic velocity and thus passes through a strong shockwave. The shock-heated gas is hot enough to generate Hα emission. Here we develop a one-dimensional radiative hydrodynamic model of the flow after the shock by detailed calculations of chemical reactions and electron transitions in hydrogen atoms, and quantify the hydrogen line emission in the Lyman-, Balmer-, and Paschen-series from the accreting gas giant system. We then demonstrate that the Hα intensity is strong enough to be detected with current observational technique. Comparing our theoretical Hα intensity with the observed one from LkCa15b, we constrain the protoplanet mass and the disk gas density. Observation of hydrogen line emission from protoplanets is highly encouraged to obtain direct constraints of accreting gas giants, which will be key in understanding the formation of gas giants.

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arXiv2017

Constraining accretion signatures of exoplanets in the TW Hya transitional disk

We present a near-infrared direct imaging search for accretion signatures of possible protoplanets around the young stellar object (YSO) TW Hya, a multi-ring disk exhibiting evidence of planet formation. The Pa$β$ line (1.282 $μ$m) is an indication of accretion onto a protoplanet, and its intensity is much higher than that of blackbody radiation from the protoplanet. We focused on the Pa$β$ line and performed Keck/OSIRIS spectroscopic observations. Although spectral differential imaging (SDI) reduction detected no accretion signatures, the results of the present study allowed us to set 5$σ$ detection limits for Pa$β$ emission of $5.8\times10^{-18}$ and $1.5\times10^{-18}$ erg/s/cm$^2$ at 0\farcs4 and 1\farcs6, respectively. We considered the mass of potential planets using theoretical simulations of circumplanetary disks and hydrogen emission. The resulting masses were $1.45\pm 0.04$ M$_{\rm J}$ and $2.29 ^{+0.03}_{-0.04}$ M$_{\rm J}$ at 25 and 95 AU, respectively, which agree with the detection limits obtained from previous broadband imaging. The detection limits should allow the identification of protoplanets as small as $\sim$1 M$_{\rm J}$, which may assist in direct imaging searches around faint YSOs for which extreme adaptive optics instruments are unavailable.

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