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Register a new userSearch for Cold Debris Disks around M-dwarfs. II
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Added by
Jean-Francois Lestrade
Publication date
2009
fields
Physics
and research's language is
English
Authors
J.-F. Lestrade
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Although 70 % of the stars in the Galaxy are M-dwarfs, thermal emission searches for cold debris disks have been conducted mostly for A-type and solar-type stars. We report on new lambda=1.2 mm continuum observations of thirty M-dwarfs, using the MAMBO-2 bolometer array camera at the IRAM 30m telescope. For a statistical analysis, we combine these data with our prior SCUBA and MAMBO-2 observations of 20 other M-dwarfs. Our total sample divides in M-dwarfs in moving groups, with relatively young ages, and in nearby M-dwarfs with unknown ages. Only one cold debris disk (GJ842.2) was detected significantly. We compare the implied disk abundance constraints with those found in two comparable submillimeter surveys of 10 to 190 Myr old A- and FGK-type stars. For the 19 youngest (ages less than 200 Myr) M-dwarfs in our sample, we derive a cold disk fraction of 5.3^{+10.5}_{-5.0} %, compared to 15 +/-11.5 % for FGK-stars and 22^{+33}_{-20} % for A-stars. Hence, for this age group, there is an apparent trend of fewer cold disks for later stellar types, i.e., lower star masses. Although its statistical significance is marginal, this trend is strengthened by the deeper sensitivity of observations in the M-dwarf sample. We derive a cold disk fraction of < 10 % for the older (likely a few Gyr) M-dwarfs in our sample. Finally, although inconclusively related to a debris disk, we present the complex millimeter structure found around the position of the M1.5 dwarf GJ526 in our sample.
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Cool M dwarfs outnumber sun-like G stars by ten to one in the solar neighborhood. Due to their proximity, small size, and low mass, M-dwarf stars are becoming attractive targets for exoplanet searches via almost all current search methods. But what planetary systems can form around M dwarfs? Following up on the Cool Stars~16 Splinter Session Planet Formation Around M Dwarfs, we summarize here our knowledge of protoplanetary disks around cool stars, how they disperse, what planetary systems might form and can be detected with current and future instruments.
Spectroscopic observations of some metal-rich white dwarfs (WDs), believed to be polluted by planetary material, reveal the presence of compact gaseous metallic disks orbiting them. The observed variability of asymmetric, double-peaked emission line profiles in about half of such systems could be interpreted as the signature of precession of an eccentric gaseous debris disk. The variability timescales --- from decades down to $1.4$ yr (recently inferred for the debris disk around HE 1349--2305) --- are in rough agreement with the rate of general relativistic (GR) precession in the test particle limit. However, it has not been demonstrated that this mechanism can drive such a fast, coherent precession of a radially extended (out to $1 R_odot$) gaseous disk mediated by internal stresses (pressure). Here we use the linear theory of eccentricity evolution in hydrodynamic disks to determine several key properties of eccentric modes in gaseous debris disks around WDs. We find a critical dependence of both the precession period and radial eccentricity distribution of the modes on the inner disk radius, $r_mathrm{in}$. For small inner radii, $r_mathrm{in} lesssim (0.2 - 0.4) R_odot$, the modes are GR-driven, with periods of $approx 1 - 10$ yr. For $r_mathrm{in} gtrsim (0.2 - 0.4) R_odot$, the modes are pressure-dominated, with periods of $approx 3 - 20$ yr. Correspondence between the variability periods and inferred inner radii of the observed disks is in general agreement with this trend. In particular, the short period of HE 1349--2305 is consistent with its small $r_mathrm{in}$. Circum-WD debris disks may thus serve as natural laboratories for studying the evolution of eccentric gaseous disks.
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 emission 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.
We report on our searches for debris disks around seven relatively nearby radio pulsars, which are isolated sources and were carefully selected as the targets on the basis of our deep $K_s$-band imaging survey. The $K_s$ images obtained with the 6.5,m Baade Magellan Telescope at Las Campanas Observatory are analyzed together with the textit{Spitzer}/IRAC images at 4.5 and 8.0~$mu$m and the textit{WISE} images at 3.4, 4.6, 12 and 22~$mu$m. No infrared (IR) counterparts of these pulsars are found, with flux upper limits of $sim mu$Jy at near-infrared ($lambda<10 mu$m) and $sim$10--1000,$mu$Jy at mid-infrared wavelengths ($lambda>10 mu$m). The results of this search are discussed in terms of the efficiency of converting the pulsar spin-down energy to thermal energy and X-ray heating of debris disks, with comparison made to the two magnetars 4U~0142+61 and 1E~2259+586 which are suggested to harbor a debris disk.
Cold debris disks (T$<$200 K) are analogues to the dust in the Solar Systems Kuiper belt--dust generated from the evaporation and collision of minor bodies perturbed by planets, our Sun, and the local interstellar medium. Scattered light from debris disks acts as both a signpost for unseen planets as well as a source of contamination for directly imaging terrestrial planets, but many details of these disks are poorly understood. We lay out a critical observational path for the study of nearby debris disks that focuses on defining an empirical relationship between scattered light and thermal emission from a disk, probing the dynamics and properties of debris disks, and directly determining the influence of planets on disks. We endorse the findings and recommendations published in the National Academy reports on Exoplanet Science Strategy and Astrobiology Strategy for the Search for Life in the Universe. This white paper extends and complements the material presented therein with a focus on debris disks around nearby stars. Separate complementary papers are being submitted regarding the inner warm regions of debris disks (Mennesson et al.), the modeling of debris disk evolution (Gaspar et al.), studies of dust properties (Chen et al.), and thermal emission from disks (Su et al.).
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