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تسجيل مستخدم جديدDiscovery of an Edge-on Circumstellar Debris Disk Around BD+45$^{circ}$598: a Newly Identifed Member of the $beta$ Pictoris Moving Group
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We report the discovery of a circumstellar debris disk viewed nearly edge-on and associated with the young, K1 star BD+45$^{circ}$598 using high-contrast imaging at 2.2$mu$m obtained at the W.M.~Keck Observatory. We detect the disk in scattered light with a peak significance of $sim$5$sigma$ over three epochs, and our best-fit model of the disk is an almost edge-on $sim$70 AU ring, with inclination angle $sim$87$^circ$. Using the NOEMA interferometer at the Plateau de Bure Observatory operating at 1.3mm, we find resolved continuum emission aligned with the ring structure seen in the 2.2$mu$m images. We estimate a fractional infrared luminosity of $L_{IR}/L_{tot}$ $simeq6^{+2}_{-1}$$times$$10^{-4}$, higher than that of the debris disk around AU Mic. Several characteristics of BD+45$^{circ}$598, such as its galactic space motion, placement in a color-magnitude diagram, and strong presence of Lithium, are all consistent with its membership in the $beta$ Pictoris Moving Group with an age of 23$pm$3 Myr. However, the galactic position for BD+45$^{circ}$598 is slightly discrepant from previously-known members of the $beta$ Pictoris Moving Group, possibly indicating an extension of members of this moving group to distances of at least 70pc. BD+45$^{circ}$598 appears to be an example from a population of young circumstellar debris systems associated with newly identified members of young moving groups that can be imaged in scattered light, key objects for mapping out the early evolution of planetary systems from $sim$10-100 Myr. This target will also be ideal for northern-hemisphere, high-contrast imaging platforms to search for self-luminous, planetary mass companions residing in this system.
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The Beta Pictoris Moving Group is a nearby stellar association of young (12Myr) co-moving stars including the classical debris disk star beta Pictoris. Due to their proximity and youth they are excellent targets when searching for submillimetre emiss
ion from cold, extended, dust components produced by collisions in Kuiper-Belt-like disks. They also allow an age independent study of debris disk properties as a function of other stellar parameters. We observed 7 infrared-excess stars in the Beta Pictoris Moving Group with the LABOCA bolometer array, operating at a central wavelength of 870 micron at the 12-m submillimetre telescope APEX. The main emission at these wavelengths comes from large, cold dust grains, which constitute the main part of the total dust mass, and hence, for an optically thin case, make better estimates on the total dust mass than earlier infrared observations. Fitting the spectral energy distribution with combined optical and infrared photometry gives information on the temperature and radial extent of the disk. From our sample, beta Pic, HD181327, and HD172555 were detected with at least 3-sigma certainty, while all others are below 2-sigma and considered non-detections. The image of beta Pic shows an offset flux density peak located near the south-west extension of the disk, similar to the one previously found by SCUBA at the JCMT. We present SED fits for detected sources and give an upper limit on the dust mass for undetected ones. We find a mean fractional dust luminosity f_dust=11x10^{-4} at t=12Myr, which together with recent data at 100Myr suggests an f_dust propto t^{-alpha} decline of the emitting dust, with alpha > 0.8.
PSO J318.5338$-$22.8603 is an extremely-red planetary-mass object that has been identified as a candidate member of the $beta$ Pictoris moving group based on its spatial position and tangential velocity. We present a high resolution $K$-band spectrum
of PSO J318.5338$-$22.8603. Using a forward-modeling Markov Chain Monte Carlo approach, we report the first measurement of the radial velocity and $v$ sin($i$) of PSO J318.5$-$22, $-$6.0$^{+0.8}_{-1.1}$ km s$^{-1}$ and 17.5$^{+2.3}_{-2.8}$ km s$^{-1}$, respectively. We calculate the space velocity and position of PSO J318.5$-$22 and confirm that it is a member of the $beta$ Pictoris moving group. Adopting an age of 23$pm$3 Myr for PSO J318.5$-$22, we determine a mass of $8.3pm0.5$ $M_{rm{Jup}}$ and effective temperature of $1127^{+24}_{-26}$ K using evolutionary models. PSO J318.5338$-$22.8603 is intermediate in mass and temperature to the directly-imaged planets $beta$ Pictoris b and 51 Eridani b, making it an important benchmark object in the sequence of planetary-mass members of the $beta$ Pictoris moving group. Combining our $v$ sin($i$) measurement with recent photometric variability data, we constrain the inclination of PSO J318.5$-$22 to $>29^{circ}$ and its rotational period to 5-10.2 hours. The equatorial velocity of PSO J318.5$-$22 indicates that its rotation is consistent with an extrapolation of the velocity-mass relationship for solar system planets.
Context: The $beta$ Pictoris moving group is one of the most well-known young associations in the solar neighbourhood and several members are known to host circumstellar discs, planets, and comets. Measuring its age with precision is basic to study s
everal astrophysical processes such as planet formation and disc evolution which are strongly age dependent. Aims: We aim to determine a precise and accurate dynamical traceback age for the $beta$ Pictoris moving group. Methods: Our sample combines the extremely precise Gaia DR2 astrometry with ground-based radial velocities measured in an homogeneous manner. We use an updated version of our algorithm to determine dynamical ages. The new approach takes into account a robust estimate of the spatial and kinematic covariance matrices of the association to improve the sample selection process and to perform the traceback analysis. Results: We estimate a dynamical age of $18.5_{-2.4}^{+2.0}$ Myr for the $beta$ Pictoris moving group. We investigated the spatial substructure of the association at birth time and we propose the existence of a core of stars more concentrated. We also provide precise radial velocity measurements for 81 members of $beta$ Pic, including ten stars with the first determination of their radial velocities. Conclusions: Our dynamical traceback age is three times more precise than previous traceback age estimates and, more important, for the first time, reconciles the traceback age with the most recent estimates of other dynamical, lithium depletion boundary, and isochronal ages. This has been possible thanks to the excellent astrometric and spectroscopic precisions, the homogeneity of our sample, and the detailed analysis of binaries and membership.
Only 20% of old field stars have detectable debris discs, leaving open the question of what disc, if any, is present around the remaining 80%. Young moving groups allow to probe this population, since discs are expected to have been brighter early on
. This paper considers the population of F~stars in the 23~Myr-old BPMG where we find that 9/12 targets possess discs. We also analyse archival ALMA data to derive radii for 4 of the discs, presenting the first image of the 63au radius disc of HD~164249. Comparing the BPMG results to disc samples from $sim45$~Myr and $sim150$~Myr-old moving~groups, and to discs found around field stars, we find the disc incidence rate in young moving~groups is comparable to that of the BPMG and significantly higher than that of field~stars. The BPMG discs tend to be smaller than those around field~stars. However, this difference is not statistically significant due to the small number of targets. Yet, by analysing the fractional luminosity vs disc radius parameter space we find that the fractional luminosities in the populations considered drop by two orders of magnitude within the first 100~Myr. This is much faster than expected by collisional evolution, implying a decay equivalent to $1/text{age}^2$. We attribute this depletion to embedded planets which would be around 170~$M_text{earth}$ to cause a depletion on the appropriate timescale. However, we cannot rule out that different birth environments of nearby young clusters result in brighter debris discs than the progenitors of field~stars which likely formed in a more dense environment.
Jeffries & Binks (2014) and Malo et al. (2014) have recently reported Li depletion boundary (LDB) ages for the {beta} Pictoris moving group (BPMG) which are twice as old as the oft-cited kinematic age of $sim$12 Myr. In this study we present (1) a ne
w evaluation of the internal kinematics of the BPMG using the revised Hipparcos astrometry and best available published radial velocities, and assess whether a useful kinematic age can be derived, and (2) derive an isochronal age based on the placement of the A-, F- and G-type stars in the colour-magnitude diagram (CMD). We explore the kinematics of the BPMG looking at velocity trends along Galactic axes, and conducting traceback analyses assuming linear trajectories, epicyclic orbit approximation, and orbit integration using a realistic gravitational potential. None of the methodologies yield a kinematic age with small uncertainties using modern velocity data. Expansion in the Galactic X and Y directions is significant only at the 1.7{sigma} and 2.7{sigma} levels, and together yields an overall kinematic age with a wide range (13-58 Myr; 95 per cent CL). The A-type members are all on the zero age-main-sequence, suggestive of an age of $>$20Myr, and the loci of the CMD positions for the late-F- and G-type pre-main-sequence BPMG members have a median isochronal age of 22 Myr ($pm$ 3 Myr stat., $pm$ 1 Myr sys.) when considering four sets of modern theoretical isochrones. The results from recent LDB and isochronal age analyses are now in agreement with a median BPMG age of 23 $pm$ 3 Myr (overall 1{sigma} uncertainty, including $pm$2 Myr statistical and $pm$2 Myr systematic uncertainties).
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