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Register a new userA search for accreting young companions embedded in circumstellar disks: High-contrast H$alpha$ imaging with VLT/SPHERE
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G. Cugno
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Aims: We want to detect and quantify observables related to accretion processes occurring locally in circumstellar disks, which could be attributed to young forming planets. We focus on objects known to host protoplanet candidates and/or disk structures thought to be the result of interactions with planets. Methods: We analyzed observations of 6 young stars (age $3.5-10$ Myr) and their surrounding environments with the SPHERE/ZIMPOL instrument on the VLT in the H$alpha$ filter (656 nm) and a nearby continuum filter (644.9 nm). Results: We re-detect the known accreting M-star companion HD142527 B with the highest published signal to noise to date in both H$alpha$ and the continuum. We derive new astrometry ($r = 62.8^{+2.1}_{-2.7}$ mas and $text{PA} = (98.7,pm1.8)^circ$) and photometry ($Delta$N_Ha=$6.3^{+0.2}_{-0.3}$ mag, $Delta$B_Ha=$6.7pm0.2$ mag and $Delta$Cnt_Ha=$7.3^{+0.3}_{-0.2}$ mag) for the companion in agreement with previous studies, and estimate its mass accretion rate ($dot{M}approx1-2,times10^{-10},M_odottext{ yr}^{-1}$). A faint point-like source around HD135344 B (SAO206462) is also investigated, but a second deeper observation is required to reveal its nature. No other companions are detected. In the framework of our assumptions we estimate detection limits at the locations of companion candidates around HD100546, HD169142 and MWC758 and calculate that processes involving H$alpha$ fluxes larger than $sim8times10^{-14}-10^{-15},text{erg/s/cm}^2$ ($dot{M}>10^{-10}-10^{-12},M_odottext{ yr}^{-1}$) can be excluded. Furthermore, flux upper limits of $sim10^{-14}-10^{-15},text{erg/s/cm}^2$ ($dot{M}<10^{-11}-10^{-12},M_odot text{ yr}^{-1}$) are estimated within the gaps identified in the disks surrounding HD135344B and TW Hya.
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The planets HR8799bc display nearly identical colours and spectra as variable young exoplanet analogues such as VHS 1256-1257ABb and PSO J318.5-22, and are likely to be similarly variable. Here we present results from a 5-epoch SPHERE IRDIS broadband-$H$ search for variability in these two planets. HR 8799b aperture photometry and HR 8799bc negative simulated planet photometry share similar trends within uncertainties. Satellite spot lightcurves share the same trends as the planet lightcurves in the August 2018 epochs, but diverge in the October 2017 epochs. We consider $Delta(mag)_{b} - Delta(mag)_{c}$ to trace non-shared variations between the two planets, and rule out non-shared variability in $Delta(mag)_{b} - Delta(mag)_{c}$ to the 10-20$%$ level over 4-5 hours. To quantify our sensitivity to variability, we simulate variable lightcurves by inserting and retrieving a suite of simulated planets at similar radii from the star as HR 8799bc, but offset in position angle. For HR 8799b, for periods $<$10 hours, we are sensitive to variability with amplitude $>5%$. For HR 8799c, our sensitivity is limited to variability $>25%$ for similar periods.
MWC758 is a young star surrounded by a transitional disk. Recently, a protoplanet candidate has been detected around MWC758 through high-resolution $L$-band observations. The candidate is located inside the disk cavity at a separation of $sim$111 mas from the central star, and at an average position angle of $sim$165.5 degrees. We have performed simultaneous adaptive optics observations of MWC758 in the H$_{alpha}$ line and the adjacent continuum using SPHERE/ZIMPOL at the Very Large Telescope (VLT). We aim at detecting accreting protoplanet candidates through spectral angular differential imaging observations. The data analysis does not reveal any H$_{alpha}$ signal around the target. The derived contrast curve in the B_Ha filter allows us to derive a 5$sigma$ upper limit of $sim$7.6 mag at 111 mas, the separation of the previously detected planet candidate. This contrast translates into a H$_{alpha}$ line luminosity of $L_{rm H_{alpha}}lesssim$ 5$times$10$^{-5}$ $L_{odot}$ at 111 mas, and an accretion luminosity of $L_{acc} <$3.7$times$10$^{-4},L_{odot}$. For the predicted mass range of MWC758b, 0.5-5 $M_{rm Jup}$, this implies accretion rates of $dot M lesssim$ 3.4$times$(10$^{-8}$-10$^{-9}),M_{odot}/yr$, for an average planet radius of 1.1 $R_{rm Jup}$. Therefore, our estimates are consistent with the predictions of accreting circumplanetary accretion models for $R_{rm in} = 1 R_{rm Jup}$. In any case, the non-detection of any H$_{alpha}$ emitting source in the ZIMPOL images does not allow us to unveil the true nature of the $L$ detected source.
Debris disk stars are good targets for high contrast imaging searches for planetary systems, since debris disks have been shown to have a tentative correlation with giant planets. We selected 20 stars identified as debris disk hosts by the WSIE mission, with particularly high levels of warm dust. We observed these with the VLT/SPHERE high contrast imaging instrument with the goal of finding planets and imaging the disks in scattered light. Our survey reaches a median 5$sigma$~sensitivity of 10.4Mj at 25au and 5.9Mj at 100au. We identified three new stellar companions (HD18378B, HD19257B and HD133778B): two are mid-M type stars and one is late-K or early-M star. Three additional stars have very widely separated stellar companions (all at $>$2000au) identified in the Gaia catalog. The stars hosting the three SPHERE-identified companions are all older ($gtrsim$700Myr), with one having recently left the main sequence and one a giant star. We infer that the high volumes of dust observed around these stars might have been caused by a recent collision between the planets and planetesimal belts in the system, although for the most evolved star, mass loss could also be responsible for the infrared excess. Future mid-IR spectroscopy or polarimetric imaging may allow the positions and spatial extent of these dust belts to be constrained, thereby providing evidence as to the true cause of the elevated levels of dust around these old systems. None of the disks in this survey are resolved in scattered light.
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We present current status of H$alpha$ high-contrast imaging observations with Subaru/SCExAO+VAMPIRES. Our adaptive optics correction at optical wavelengths in combination with (double) spectral differential imaging (SDI) and angular differential imaging (ADI) was capable of detecting a ring-like feature around omi Cet and the H$alpha$ counterpart of jet around RY Tau. We tested the post-processing by changing the order of ADI and SDI and both of the contrast limits achieved $sim10^{-3}-5times10^{-4}$ at $0.3^{primeprime}$, which is comparable to other H$alpha$ high-contrast imaging instruments in the southern hemisphere such as VLT/SPHERE, VLT/MUSE, and MagAO. Subaru/VAMPIRES provides great opportunities for H$alpha$ high-contrast imaging for northern hemisphere targets.
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