Stripping a debris disk by close stellar encounters in an open stellar cluster

A debris disk is a constituent of any planetary system surrounding a main sequence star. We study whether close stellar encounters can disrupt and strip a debris disk of its planetesimals in the expanding open cluster of its birth with a decreasing star number density over 100 Myrs. Such stripping would affect the dust production and hence detectability of the disk. We tabulated the fractions of planetesimals stripped off during stellar flybys of miss distances between 100 and 1000 AU and for several mass ratios of the central to passing stars. We then estimated the numbers of close stellar encounters over the lifetime of several expanding open clusters characterized by their initial star densities. We found that a standard disk, with inner and outer radii of 40 and 100 AU, suffers no loss of planetesimals over 100 Myrs around a star born in a common embedded cluster with star density <1000 pc^-3. In contrast, we found that such a disk is severely depleted of its planetesimals over this timescale around a star born in an Orion-type cluster where the star density is >20 000 pc^-3. In this environment, a disk loses >97% of its planetesimals around an M-dwarf, >63% around a solar-type star, and >42% around an A-dwarf, over 100 Myrs. We roughly estimate that two-thirds of the stars may be born in such high star density clusters. This might explain in part why fewer debris disks are observed around lower mass stars.Comment: 7 pages, 4 figures, accepted for publication in Astronomy and Astrophysics ; v2 abstract complemente

Resolving debris discs in the far-infrared: early highlights from the DEBRIS survey

We present results from the earliest observations of DEBRIS, a Herschel Key Programme to conduct a volume- and flux-limited survey for debris discs in A-type through M-type stars. PACS images (from chop/nod or scan-mode observations) at 100 and 160 micron are presented toward two A-type stars and one F-type star: beta Leo, beta UMa and eta Corvi. All three stars are known disc hosts. Herschel spatially resolves the dust emission around all three stars (marginally, in the case of beta UMa), providing new information about discs as close as 11 pc with sizes comparable to that of the Solar System. We have combined these data with existing flux density measurements of the discs to refine the SEDs and derive estimates of the fractional luminosities, temperatures and radii of the discs.Comment: to be published in A&A, 5 pages, 2 color figure

ALMA’s view of the M-dwarf GSC 07396-00759’s edge-on debris disc : AU Mic’s coeval twin

We present new ALMA Band 7 observations of the edge-on debris disc around the M1V star GSC 07396-00759. At ~20 Myr old and in the β Pictoris Moving Group along with AU Mic, GSC 07396-00759 joins it in the handful of low mass M-dwarf discs to be resolved in the sub-mm. With previous VLT/SPHERE scattered light observations we present a multi-wavelength view of the dust distribution within the system under the effects of stellar wind forces. We find the mm dust grains to be well described by a Gaussian torus at 70 au with a FWHM of 48 au and we do not detect the presence of CO in the system. Our ALMA model radius is significantly smaller than the radius derived from polarimetric scattered light observations, implying complex behaviour in the scattering phase function. The brightness asymmetry in the disc observed in scattered light is not recovered in the ALMA observations, implying that the physical mechanism only affects smaller grain sizes. High resolution follow-up observations of the system would allow investigation into its unique dust features as well as provide a true coeval comparison for its smaller sibling AU Mic, singularly well observed amongst M-dwarfs systems

The REASONS Survey : resolved millimeter observations of a large debris disk around the nearby F Star HD 170773

Debris disks are extrasolar analogs to our own Kuiper Belt and they are detected around at least 17% of nearby Sun-like stars. The morphology and dynamics of a disk encode information about its history, as well as that of any exoplanets within the system. We used the Atacama Large Millimeter/submillimeter Array (ALMA) to obtain 1.3 mm observations of the debris disk around the nearby F5V star HD 170773. We image the face-on ring and determine its fundamental parameters by forward-modeling the interferometric visibilities through a Markov Chain Monte Carlo approach. Using a symmetric Gaussian surface density profile, we find a 71 ± 4 au wide belt with a radius of {193}-3+2 au, a relatively large radius compared with most other millimeter-resolved belts around late A/early F type stars. This makes HD 170773 part of a group of four disks around A and F stars with radii larger than expected from the recently reported planetesimal belt radius—stellar luminosity relation. Two of these systems are known to host directly imaged giant planets, which may point to a connection between large belts and the presence of long-period giant planets. We also set upper limits on the presence of CO and CN gas in the system, which imply that the exocomets that constitute this belt have CO and HCN ice mass fractions of <77% and <3%, respectively. This is consistent with solar system comets and other exocometary belts

Dispersion measure variations observed in the direction of the millisecond pulsar PSR B1821-24.

International audienceHigh-precision timing observations of the millisecond pulsar PSR B1821-24 have been conducted at the Nancay radiotelescope between 1.37GHz and 1.7GHz since January 22, 1992 until January 10, 1996. This dense and precise multi-frequency timing series has allowed secular variations of the dispersion measure as high as 0.005pc/cm^3^/year in the direction of this pulsar to be determined. The index β of the power-law spatial spectrum of the electron density fluctuations has been determined for the first time in the direction of this pulsar and is 3.727+/-0.211, a result consistent with a Kolmogorov turbulent medium. The diffractive time scale τ_d_ is also determined to be 50+/-_ 40_^183^ s and is relatively consistent with the determination based on the space velocity of PSR B1821-24. We also provide a new proper motion for this pulsar for which the declination component is for the first time inconsistent with zero

On the steady state collisional evolution of debris disks around M dwarfs

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Interstellar modulation of the flux density and arrival time of pulses from pulsar B 1937+214

International audienceObservations of the millisecond pulsar B1937+214 made at Nan\c cay over 6 years show 30% rms flux variations over 13 +/- 4 days due to Refractive Interstellar Scintillations. The arrival times (TOA) also show variations over a similar time scale 16 +/- 10 days with an rms amplitude of about 0.3mu secs. These ``rapid'' TOA variations are anti-correlated ( ~ -40%) with the flux and so are also caused by propagation through the ionized interstellar medium. The correlation is such that weak pulses tend to arrive late. While TOA modulations due to changing geometric delay should be positively correlated with flux, those due to small scale variations in the dispersive delay should be negatively correlated with the flux and so are presumed to be responsible in our observations. The level and time scales are shown to be consistent with expectations based on the Kolmogorov model of the interstellar density spectrum. However, in the data there is a sequence of about 5 discrete events, in which the flux remains low over 10-30 days and the TOA is on average late but also shows rapid variations. Assuming that these are indeed discrete events, we interpret them as due to isolated regions of enhanced plasma density crossing the line of sight. Such ``Extreme Scattering Events'' make a major contribution to the TOA variations and their anti-correlations with the observed flux. They are seen against a background of the normal refractive scintillation. A model is proposed in which discrete sheets of plasma cross the line of sight and cause a ``de-focussing'' event when aligned parallel to the line of sight. The statistics of the events imply a surprisingly large space density of the sheets; an alternative is that by chance we view PSR B1937+214 tangentially through a supernova shell which is fragmented and so causes multiple events

A 3 Year Long Extreme Scattering Event in the Direction of the Millisecond Pulsar J1643−1224

International audienceAn extreme scattering event (ESE) has been detected in the direction of the millisecond pulsar J1643-1224 at 1.28 and 1.41 GHz. Its duration is 3 years and this makes it the longest ESE ever recorded. We have used the standard model of a purely refractive lens to interpret the observed radio light curves. This lens is a fully ionized cloud crossing the line of sight. We have found that our data imply that its transverse size is 56 AU, which is much larger than similar to1 AU typical of the other ESEs in the direction of the pulsar B1937+21. If the cloud is located at the mid-distance to J1643-1224 (2500 pc), its electron density is 130 e cm(-3). Such a highly pressurized structure has a short lifetime (29 yr), and this requires a replenishment mechanism at work in the interstellar medium that is unidentified at present. Alternative models are filamentary clouds seen through their long axis or a cold molecular hydrogen cloud with an ionized shell, as proposed by Walker Wardle

THE EPSILON ERIDANI SYSTEM RESOLVED BY MILLIMETER INTERFEROMETRY

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