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Register a new userSub-millimeter non-contaminated detection of the disk around TWA,7 by ALMA
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Debris disks can be seen as the left-overs of giant planet formation and the possible nurseries of rocky planets. While M-type stars out-number more massive stars we know very little about the time evolution of their circumstellar disks at ages older than $sim 10$,Myr. Sub-millimeter observations are best to provide first order estimates of the available mass reservoir and thus better constrain the evolution of such disks. Here, we present ALMA Cycle,3 Band,7 observations of the debris disk around the M2 star TWA,7, which had been postulated to harbor two spatially separated dust belts, based on unresolved far-infrared and sub-millimeter data. We show that most of the emission at wavelengths longer than $sim 300$,$mu$m is in fact arising from a contaminant source, most likely a sub-mm galaxy, located at about 6.6 East of TWA,7 (in 2016). Fortunately, the high resolution of our ALMA data allows us to disentangle the contaminant emission from that of the disc and report a significant detection of the disk in the sub-millimeter for the first time with a flux density of 2.1$pm$0.4 mJy at 870 $mu$m. With this detection, we show that the SED can be reproduced with a single dust belt.
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IRS 7 is an M red supergiant star which is located at $5.5$ north of Sagittarius A$^ast$. We detected firstly the continuum emission at 340 GHz of IRS 7 using ALMA. The total flux density of IRS 7 is $S_ u=448pm45 mu$Jy. The flux density indicates that IRS 7 has a photosphere radius of $R=1170pm60 ~R_odot$, which is roughly consistent with the previous VLTI measurement. We also detected a shell like feature with north extension in the H30$alpha$ recombination line by ALMA. The electron temperature and electron density of the shell like structure are estimated to be $bar{T}^ast_{mathrm e}=4650pm500$ K and $bar{n}_{mathrm e}=(6.1pm0.6)times10^4$ cm$^{-3}$, respectively. The mass loss rate is estimated to be $dot{m} sim 1times 10^{-4} M_odot$ yr$^{-1}$, which is consistent with a typical mass loss rate of a pulsating red supergiant star with $M=20-25 M_odot$. The kinematics of the ionized gas would support the hypothesis that the shell like structure made by the mass loss of IRS 7 is supersonically traveling in the ambient matter toward the south. The brightened southern half of the structure and the north extension would be a bow shock and a cometary-like tail structure, respectively.
We present the first images of four debris disks observed in scattered light around the young (4--250 Myr old) M dwarfs TWA 7 and TWA 25, the K6 star HD 35650, and the G2 star HD 377. We obtained these images by reprocessing archival Hubble Space Telescope NICMOS coronagraph data with modern post-processing techniques as part of the Archival Legacy Investigation of Circumstellar Environments (ALICE) program. All four disks appear faint and compact compared with other debris disks resolved in scattered light. The disks around TWA 25, HD 35650, and HD 377 appear very inclined, while TWA 7s disk is viewed nearly face-on. The surface brightness of HD 35650s disk is strongly asymmetric. These new detections raise the number of disks resolved in scattered light around M and late-K stars from one (the AU Mic system) to four. This new sample of resolved disks enables comparative studies of heretofore scarce debris disks around low-mass stars relative to solar-type stars.
OTS44 is one of only four free-floating planets known to have a disk. We have previously shown that it is the coolest and least massive known free-floating planet ($sim$12 M$_{rm Jup}$) with a substantial disk that is actively accreting. We have obtained Band 6 (233 GHz) ALMA continuum data of this very young disk-bearing object. The data shows a clear unresolved detection of the source. We obtained disk-mass estimates via empirical correlations derived for young, higher-mass, central (substellar) objects. The range of values obtained are between 0.07 and 0.63 M$_{oplus}$ (dust masses). We compare the properties of this unique disk with those recently reported around higher-mass (brown dwarfs) young objects in order to infer constraints on its mechanism of formation. While extreme assumptions on dust temperature yield disk-mass values that could slightly diverge from the general trends found for more massive brown dwarfs, a range of sensible values provide disk masses compatible with a unique scaling relation between $M_{rm dust}$ and $M_{*}$ through the substellar domain down to planetary masses.
Using HST-COS FUV spectra, we have discovered warm molecular hydrogen in the TWA 7 system. TWA 7, a $sim$9 Myr old M2.5 star, has a cold debris disk and has previously shown no signs of accretion. Molecular hydrogen is expected to be extremely rare in a debris disk. While molecular hydrogen can be produced in star spots or the lower chromospheres of cool stars such as TWA 7, fluxes from progressions that get pumped by the wings of Ly$alpha$ indicate that this molecular hydrogen could be circumstellar and thus that TWA 7 is accreting at very low levels and may retain a reservoir of gas in the near circumstellar environment.
We have observed the Class I protostar L1489 IRS with the Atacama Millimeter/submillimeter Array (ALMA) in Band 6. The C$^{18}$O $J=$2-1 line emission shows flattened and non-axisymmetric structures in the same direction as its velocity gradient due to rotation. We discovered that the C$^{18}$O emission shows dips at a radius of ~200-300 au while the 1.3 mm continuum emission extends smoothly up to r~400 au. At the radius of the C$^{18}$O dips, the rotational axis of the outer portion appears to be tilted by ~15 degrees from that of the inner component. Both the inner and outer components with respect to the C$^{18}$O dips exhibit the $r^{-0.5}$ Keplerian rotation profiles until r~600 au. These results not only indicate that a Keplerian disk extends up to ~600 au but also that the disk is warped. We constructed a three dimensional warped disk model rotating at the Keplerian velocity, and demonstrated that the warped disk model reproduces main observed features in the velocity channel maps and the PV diagrams. Such a warped disk system can form by mass accretion from a misaligned envelope. We also discuss a possible disk evolution scenario based on comparisons of disk radii and masses between Class I and Class II sources.
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