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Register a new userA Search for Cold Dust around Neutron Stars
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We present observations of nine radio pulsars using the Heinrich-Hertz-Telescope at lambda 0.87mm and the IRAM 30-m telescope at lambda 1.2mm in search for a cold dust around these sources. Five of the program pulsars have been observed for the first time at the mm-wavelengths. The results are consistent with the absence of circumpulsar disks that would be massive enough ($ge 0.01 M_{odot}$) to support planet formation according to the scenarios envisioned for solar-type stars, but they do not exclude lower mass ($le 10-100 M_{oplus}$) disks for a wide range of grain sizes. These conclusions confirm the previously published results and, together with the current lack of further detections of pulsar planets, they suggest that planet formation around neutron stars is not a common phenomenon.
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Globular cluster stars evolving off the main sequence are known to lose mass, and it is expected that some of the lost material should remain within the cluster as an intracluster medium (ICM). Most attempts to detect such an ICM have been unsuccessful. The Multiband Imaging Photometer for Spitzer on the Spitzer Space Telescope was used to observe eight Galactic globular clusters in an attempt to detect the thermal emission from ICM dust. Most clusters do not have significant detections at 70 microns; one cluster, NGC 6341, has tentative evidence for the presence of dust, but 90 micron observations do not confirm the detection. Individual 70 micron point sources which appear in several of the cluster images are likely to be background galaxies. The inferred dust mass and upper limits are < 4e-4 solar masses, well below expectations for cluster dust production from mass loss in red and asymptotic giant branch stars. This implies that either globular cluster dust production is less efficient, or that ICM removal or dust destruction is more efficient, than previously believed. We explore several possibilities for ICM removal and conclude that present data do not yet permit us to distinguish between them.
We report the complete photometric results from our Herschel study which is the first comprehensive program to search for far-infrared emission from cold dust around young brown dwarfs. We surveyed 50 fields containing 51 known or suspected brown dwarfs and very low mass stars that have evidence of circumstellar disks based on Spitzer photometry and/or spectroscopy. The objects with known spectral types range from M3 to M9.5. Four of the candidates were subsequently identified as extragalactic objects. Of the remaining 47 we have successfully detected 36 at 70micron and 14 at 160micron with S/N greater than 3, as well as several additional possible detections with low S/N. The objects exhibit a range of [24]--[70] micron colors suggesting a range in mass and/or structure of the outer disk. We present modeling of the spectral energy distributions of the sample and discuss trends visible in the data. Using two Monte Carlo radiative transfer codes we investigate disk masses and geometry. We find a very wide range in modeled total disk masses from less than 1e-6 solar masses up to 1e-3 solar masses with a median disk mass of order 3e-5 solar masses, suggesting that the median ratio of disk mass to central object mass may be lower than for T Tauri stars. The disk scale heights and flaring angles, however, cover a range consistent with those seen around T Tauri stars. The host clouds in which the young brown dwarfs and low-mass stars are located span a range in estimated age from ~1-3 Myr to ~10 Myr and represent a variety of star-forming environments. No obvious dependence on cloud location or age is seen in the disk properties, though the statistical significance of this conclusion is not strong.
We report results of a sensitive search for cold dust and molecular gas in the disks around 8 T Tauri stars in the high-latitude cloud MBM 12. Interferometric observations of 3 mm continuum emission in 5 fields containing 6 of the objects, and literature values for the remaining two, limit the disk masses to M_disk < 0.04 - 0.09 M_sun (gas+dust), for a gas:dust mass ratio of 100 and a distance of 275 pc. By coadding the 3 mm data of our five fields, we set an upper limit to the average disk mass of <M_disk>(N=5) < 0.03 M_sun. Simultaneous observation of the CS J=2-1 and the N2H+ 1-0 lines show no emission. Single-dish observations of the 13CO 2-1 line limit the disk mass to (5 - 10) * 10^-4 M_sun for a standard CO abundance of 2 * 10^-4. Depletion of CO by up to two orders of magnitude, through freezing out or photodissociation, can reconcile these limits. These mass limits lie within the range found in the Taurus-Auriga and rho Oph star-forming regions (0.001 - 0.3 M_sun), and preclude conclusions about possible decrease in disk mass over the 1 - 2 Myr age range spanned by the latter two regions and MBM 12. Our observations can exclude the presence in MBM 12 of T Tauri stars with relatively bright and massive disks such as T Tau, DG Tau, and GG Tau.
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.
We present the first far-IR observations of the solar-type stars delta Pav, HR 8501, 51 Peg and zeta^2 Ret, taken within the context of the DUNES Herschel Open Time Key Programme (OTKP). This project uses the PACS and SPIRE instruments with the objective of studying infrared excesses due to exo-Kuiper belts around nearby solar-type stars. The observed 100 um fluxes from delta Pav, HR 8501, and 51 Peg agree with the predicted photospheric fluxes, excluding debris disks brighter than Ldust/Lstar ~ 5 x 10^-7 (1 sigma level) around those stars. A flattened, disk-like structure with a semi-major axis of ~ 100 AU in size is detected around zeta^2 Ret. The resolved structure suggests the presence of an eccentric dust ring, which we interpret as an exo-Kuiper belt with Ldust/Lstar ~ 10^-5.
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