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Register a new userNew debris disks in nearby young moving groups
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A significant fraction of nearby young moving group members harbor circumstellar debris dust disks. Due to their proximity and youth, these disks are attractive targets for studying the early evolution of debris dust and planetesimal belts. Here we present 70 and 160$mu$m observations of 31 systems in the $beta$ Pic moving group, and in the Tucana-Horologium, Columba, Carina and Argus associations, using the Herschel Space Observatory. None of these stars were observed at far-infrared wavelengths before. Our Herschel measurements were complemented by photometry from the WISE satellite for the whole sample, and by submillimeter/millimeter continuum data for one source, HD 48370. We identified six stars with infrared excess, four of them are new discoveries. By combining our new findings with results from the literature, we examined the incidence and general characteristics of debris disks around Sun-like members of the selected groups. With their dust temperatures of <45 K the newly identified disks around HD 38397, HD 48370, HD 160305, and BD-20 951 represent the coldest population within this sample. For HD 38397 and HD 48370, the emission is resolved in the 70$mu$m PACS images, the estimated radius of these disks is ~90 au. Together with the well-known disk around HD 61005, these three systems represent the highest mass end of the known debris disk population around young G-type members of the selected groups. In terms of dust content, they resemble the hypothesized debris disk of the ancient Solar System.
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According to the current paradigm of circumstellar disk evolution, gas-rich primordial disks evolve into gas-poor debris disks compose of second-generation dust. To explore the transition between these phases, we searched for $^{12}$CO, $^{13}$CO, and C$^{18}$O emission in seven dust-rich debris disks around young A-type stars, using ALMA in Band 6. We discovered molecular gas in three debris disks. In all these disks, the $^{12}$CO line was optically thick, highlighting the importance of less abundant molecules in reliable mass estimates. Supplementing our target list by literature data, we compiled a volume-limited sample of dust-rich debris disks around young A-type stars within 150 pc. We obtained a CO detection rate of 11/16 above a $^{12}$CO J=2$-$1 line luminosity threshold of $sim 1.4 times 10 ^4$ Jykms$^{-1}$pc$^2$ in the sample. This high incidence implies that the presence of CO gas in bright debris disks around young A-type stars is likely more the rule than the exception. Interestingly, dust-rich debris disks around young FG-type stars exhibit, with the same detectability threshold as for A-type stars, significantly lower gas incidence. While the transition from protoplanetary to debris phase is associated with a drop of dust content, our results exhibit a large spread in the CO mass in our debris sample, with peak values comparable to those in protoplanetary Herbig Ae disks. In the particularly CO-rich debris systems the gas may have primordial origin, characteristic of a hybrid disk.
The chemical composition of gas and ice in disks around young stars set the bulk composition of planets. In contrast to protoplanetary disks (Class II), young disks that are still embedded in their natal envelope (Class 0 and I) are predicted to be too warm for CO to freeze out, as has been confirmed observationally for L1527 IRS. To establish whether young disks are generally warmer than their more evolved counterparts, we observed five young (Class 0/I and Class I) disks in Taurus with the Atacama Large Millimeter/submillimeter Array (ALMA), targeting C$^{17}$O $2-1$, H$_2$CO $3_{1,2}-2_{1,1}$, HDO $3_{1,2}-2_{2,1}$ and CH$_3$OH $5_K-4_K$ transitions at $0.48^{primeprime} times 0.31^{primeprime}$ resolution. The different freeze-out temperatures of these species allow us to derive a global temperature structure. C$^{17}$O and H$_2$CO are detected in all disks, with no signs of CO freeze-out in the inner $sim$100 au, and a CO abundance close to $sim$10$^{-4}$. H$_2$CO emission originates in the surface layers of the two edge-on disks, as witnessed by the especially beautiful V-shaped emission pattern in IRAS~04302+2247. HDO and CH$_3$OH are not detected, with column density upper limits more than 100 times lower than for hot cores. Young disks are thus found to be warmer than more evolved protoplanetary disks around solar analogues, with no CO freeze-out (or only in the outermost part of $gtrsim$100 au disks) or CO processing. However, they are not as warm as hot cores or disks around outbursting sources, and therefore do not have a large gas-phase reservoir of complex molecules.
WISEA J080822.18-644357.3, an M star in the Carina association, exhibits extreme infrared excess and accretion activity at an age greater than the expected accretion disk lifetime. We consider J0808 as the prototypical example of a class of M star accretion disks at ages $gtrsim 20$ Myr, which we call ``Peter Pan disks, since they apparently refuse to grow up. We present four new Peter Pan disk candidates identified via the Disk Detective citizen science project, coupled with textit{Gaia} astrometry. We find that WISEA J044634.16-262756.1 and WISEA J094900.65-713803.1 both exhibit significant infrared excess after accounting for nearby stars within the 2MASS beams. The J0446 system has $>95%$ likelihood of Columba membership. The J0949 system shows $>95%$ likelihood of Carina membership. We present new GMOS optical spectra of all four objects, showing possible accretion signatures on all four stars. We present ground-based and textit{TESS} lightcurves of J0808 and 2MASS J0501-4337, including a large flare and aperiodic dipping activity on J0808, and strong periodicity on J0501. We find Pa$beta$ and Br$gamma$ emission indicating ongoing accretion in near-IR spectroscopy of J0808. Using observed characteristics of these systems, we discuss mechanisms that lead to accretion disks at ages $gtrsim20$ Myr, and find that these objects most plausibly represent long-lived CO-poor primordial disks, or ``hybrid disks, exhibiting both debris- and primordial-disk features. The question remains: why have gas-rich disks persisted so long around these particular stars?
The detection of gas in debris disks raises the question of whether this gas is a remnant from the primordial protoplanetary phase, or released by the collision of secondary bodies. In this paper we analyze ALMA observations at 1-1.5 resolution of three debris disks where the $^{12}$CO(2-1) rotational line was detected: HD131835, HD138813, and HD156623. We apply the iterative Lucy-Richardson deconvolution technique to the problem of circumstellar disks to derive disk geometries and surface brightness distributions of the gas. The derived disk parameters are used as input for thermochemical models to test both primordial and cometary scenarios for the origin of the gas. We favor a secondary origin for the gas in these disks and find that the CO gas masses ($sim 3times10^{-3}$ M$_{oplus}$) require production rates ($sim 5times 10^{-7}$ M$_{oplus}$~yr$^{-1}$) similar to those estimated for the bona-fide gas rich debris disk $beta$ Pic.
Context. Studying gas chemistry in protoplanetary disks is key to understanding the process of planet formation. Sulfur chemistry in particular is poorly understood in interstellar environments, and the location of the main reservoirs remains unknown. Protoplanetary disks in Taurus are ideal targets for studying the evolution of the composition of planet forming systems. Aims. We aim to elucidate the chemical origin of sulfur-bearing molecular emission in protoplanetary disks, with a special focus on H$_2$S emission, and to identify candidate species that could become the main molecular sulfur reservoirs in protoplanetary systems. Methods. We used IRAM 30m observations of nine gas-rich young stellar objects (YSOs) in Taurus to perform a survey of sulfur-bearing and oxygen-bearing molecular species. In this paper we present our results for the CS 3-2 ($ u_0$ = 146.969 GHz), H$_2$CO 2$_{11}$-1$_{10}$ ($ u_0$ = 150.498 GHz), and H$_2$S 1$_{10}$-1$_{01}$ ($ u_0$ = 168,763 GHz) emission lines. Results. We detected H$_2$S emission in four sources out of the nine observed, significantly increasing the number of detections toward YSOs. We also detected H$_2$CO and CS in six out of the nine. We identify a tentative correlation between H$_2$S 1$_{10}$-1$_{01}$ and H$_2$CO 2$_{11}$-1$_{10}$ as well as a tentative correlation between H$_2$S 1$_{10}$-1$_{01}$ and H$_2$O 8$_{18}$-7$_{07}$. By assuming local thermodynamical equilibrium, we computed column densities for the sources in the sample, with N(o-H$_2$S) values ranging between $2.6times10^{12}$ cm$^{-2}$ and $1.5times10^{13}$ cm$^{-2}$.
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