DENIS-P J104814.7-395606.1: An M9 dwarfs at about 4 pc

We present the discovery of a previously unknown member of the immediate solar neighbourhood, DENIS-P J104814.7-395606.1 (hereafter DENIS 1048-39), identified while mining the DENIS database for new nearby stars. A HIRES echelle spectrum obtained with the 10-m Keck telescope shows that it is an M9 dwarf. DENIS 1048-39 has a very bright apparent magnitude (I=12.67) for its spectral type and colour (I-J=3.07), and is therefore very nearby. If it is single its distance is only 4.1 +- 0.6pc, ranking it as between our twelfth and fortyth closest neighbour. It is also the closest star or brown dwarf with a spectral type later than M7V. Its proper motion was determined through comparison of Sky atlas Schmidt plates, scanned by the MAMA microdensitometer, with the DENIS images. At 1.53"/yr it further attests the closeness of DENIS 1048-39, and hence its dwarf status. These characteristics make it an obvious target for further detailed studies.Comment: In press in A&A Letter

The 1.2 Millimeter Image of the beta Pictoris Disk

We present millimeter imaging observations in the 1200 micron continuum of the disk around beta Pictoris. With the 25 arcsec beam, the beta Pic disk is unresolved perpendicularly to the disk plane (< 10 arcsec), but slightly resolved in the northeast-southwest direction (26 arcsec). Peak emission is observed at the stellar position. A secondary maximum is found 1000 AU along the disk plane in the southwest, which does not positionally coincide with a similar feature reported earlier at 850 micron. Arguments are presented which could be seen in support of the reality of these features. The observed submm/mm emission is consistent with thermal emission from dust grains, which are significantly larger than those generally found in the interstellar medium, including mm-size particles, and thus more reminiscent of the dust observed in protostellar disks. Modelling the observed scattered light in the visible and the emission in the submm/mm provides evidence for the particles dominating the scattering in the visible/NIR and those primarily responsible for the thermal emission at longer wavelengths belonging to different populations.Comment: 6 pages, 3 postscript figures, accepted for publication in Astronomy and Astrophysic

Constraining the orbit of the possible companion to Beta Pictoris: New deep imaging observations

We recently reported on the detection of a possible planetary-mass companion to Beta Pictoris at a projected separation of 8 AU from the star, using data taken in November 2003 with NaCo, the adaptive-optics system installed on the Very Large Telescope UT4. Eventhough no second epoch detection was available, there are strong arguments to favor a gravitationally bound companion rather than a background object. If confirmed and located at a physical separation of 8 AU, this young, hot (~1500 K), massive Jovian companion (~8 Mjup) would be the closest planet to its star ever imaged, could be formed via core-accretion, and could explain the main morphological and dynamical properties of the dust disk. Our goal was to return to Beta Pic five years later to obtain a second-epoch observation of the companion or, in case of a non-detection, constrain its orbit. Deep adaptive-optics L'-band direct images of Beta Pic and Ks-band Four-Quadrant-Phase-Mask (4QPM) coronagraphic images were recorded with NaCo in January and February 2009. We also use 4QPM data taken in November 2004. No point-like signal with the brightness of the companion candidate (apparent magnitudes L'=11.2 or Ks ~ 12.5) is detected at projected distances down to 6.5 AU from the star in the 2009 data. As expected, the non-detection does not allow to rule out a background object; however, we show that it is consistent with the orbital motion of a bound companion that got closer to the star since first observed in 2003 and that is just emerging from behind the star at the present epoch. We place strong constraints on the possible orbits of the companion and discuss future observing prospects.Comment: 8 pages, 8 figures, 1 table, accepted for publication in Astronomy and Astrophysic

The Dust Trail of Comet 67P/Churyumov-Gerasimenko between 2004 and 2006

We report on observations of the dust trail of comet 67P/Churyumov-Gerasimenko (CG) in visible light with the Wide Field Imager at the ESO/MPG 2.2m telescope at 4.7 AU before aphelion, and at 24 micron with the MIPS instrument on board the Spitzer Space Telescope at 5.7 AU both before and after aphelion. The comet did not appear to be active during our observations. Our images probe large dust grains emitted from the comet that have a radiation pressure parameter beta<0.01. We compare our observations with simulated images generated with a dynamical model of the cometary dust and constrain the emission speeds, size distribution, production rate and geometric albedo of the dust. We achieve the best fit to our data with a differential size distribution exponent of -4.1, and emission speeds for a beta=0.01 particle of 25 m/s at perihelion and 2 m/s at 3 AU. The dust production rate in our model is on the order of 1000 kg/s at perihelion and 1 kg/s at 3 AU, and we require a dust geometric albedo between 0.022 and 0.044. The production rates of large (>10 micron) particles required to reproduce the brightness of the trail are sufficient to also account for the coma brightness observed while the comet was inside 3 AU, and we infer that the cross-section in the coma of CG may be dominated by grains of the order of 60-600 micron.Comment: 79 pages, 13 figures, 6 tables. Accepted for publication in Icaru

Discovery of a nearby M9 dwarf

We report the discovery of a new M9.0 dwarf at only 8.2 pc, which we identified in our search for nearby ultracool dwarf (I-J >= 3.0, later than M8.0) in the DENIS database. We measure a very high proper motion of 2.5 arc-sec/yr. The PC3 index measured from its low-resolution spectrum gives a spectrophotometric distance of 8.2 pc. This makes it the third closest M9.0 dwarf.Comment: revised version, accepted by MNRAS Letter

Explosion of Comet 17P/Holmes as revealed by the Spitzer Space Telescope

An explosion on comet 17P/Holmes occurred on 2007 Oct 23, projecting particulate debris of a wide range of sizes into the interplanetary medium. We observed the comet using the Spitzer spectrograph on 2007 Nov 10 and 2008 Feb 27, and the photometer, on 2008 Mar 13. The fresh ejecta have detailed mineralogical features from small crystalline silicate grains. The 2008 Feb 27 spectra, and the central core of the 2007 Nov 10 spectral map, reveal nearly featureless spectra, due to much larger grains that were ejected from the nucleus more slowly. We break the infrared image into three components (size, speed) that also explain the temporal evolution of the mm-wave flux. Optical images were obtained on multiple dates spanning 2007 Oct 27 to 2008 Mar 10 at the Holloway Comet Observatory and 1.5-m telescope at Palomar Observatory. The orientation of the leading edge of the ejecta shell and the ejecta blob, relative to the nucleus, do not change as the orientation of the Sun changes; instead, the configuration was imprinted by the orientation of the initial explosion. The kinetic energy of the ejecta >1e21 erg is greater than the gravitational binding energy of the nucleus. We model the explosion as being due to crystallization and release of volatiles from interior amorphous ice within a subsurface cavity; once the pressure in the cavity exceeded the surface strength, the material above the cavity was propelled from the comet. The size of the cavity and the tensile strength of the upper layer of the nucleus are constrained by the observed properties of the ejecta; tensile strengths on >10 m scale must be greater than 10 kPa. The appearance of the 2007 outburst is similar to that witnessed in 1892, but the 1892 explosion was less energetic by a factor of about 20.Comment: 51 pages. Some figures compressed (see real journal for full quality). accepted by Icaru

Spectroscopic survey of the Galaxy with Gaia I. Design and performance of the Radial Velocity Spectrometer

The definition and optimisation studies for the Gaia satellite spectrograph, the Radial Velocity Spectrometer (RVS), converged in late 2002 with the adoption of the instrument baseline. This paper reviews the characteristics of the selected configuration and presents its expected performance. The RVS is a 2.0 by 1.6 degree integral field spectrograph, dispersing the light of all sources entering its field of view with a resolving power R=11 500 over the wavelength range [848, 874] nm. The RVS will continuously and repeatedly scan the sky during the 5 years of the Gaia mission. On average, each source will be observed 102 times over this period. The RVS will collect the spectra of about 100-150 million stars up to magnitude V~17-18. At the end of the mission, the RVS will provide radial velocities with precisions of ~2 km/s at V=15 and \~15-20 km/s at V=17, for a solar metallicity G5 dwarf. The RVS will also provide rotational velocities, with precisions (at the end of the mission) for late type stars of sigma_vsini ~5 km/s at V~15 as well as atmospheric parameters up to V~14-15. The individual abundances of elements such as Silicon and Magnesium, vital for the understanding of Galactic evolution, will be obtained up to V~12-13. Finally, the presence of the 862.0 nm Diffuse Interstellar Band (DIB) in the RVS wavelength range will make it possible to derive the three dimensional structure of the interstellar reddening.Comment: 17 pages, 9 figures, accepted for publication in MNRAS. Fig. 1,2,4,5, 6 in degraded resolution; available in full resolution at http://blackwell-synergy.com/links/doi/10.1111/j.1365-2966.2004.08282.x/pd

New neighbours: V. 35 DENIS late-M dwarfs between 10 and 30 parsecs

This paper reports updated results on our systematic mining of the DENIS database for nearby very cool M-dwarfs (M6V-M8V, 2.0<I-J<3.0, photometric distance within 30 pc), We calibrate the DENIS (M_I, I-J) colour-luminosity relationship from M dwarfs with well measured parallaxes (HIP, GCTP,...), obtaining distance errors for single dwarfs of 25%. Using proper motions measured on archive Schmidt plates for stars that meet the photometric selection criteria, we eliminate the giants by a Reduced Proper Motion cutoff, which is significantly more selective than a simple proper motion cutoff. Here we present new data for 62 red dwarf candidates selected over 5700 square degrees in the DENIS database. 26 of those originate in the 2100 square degrees analysed in Paper I, with improved parameters here, and 36 were found in 3600 additional square degrees. 25 of those are new nearby dwarfs. We determine from that sample of 62 stars a stellar density for 12.0<M_I<14.0 of 2.2(+-0.4)E-3 stars/pc^3/mag. This value is consistent with photometric luminosity functions measured from deeper and smaller-field observations, but not with the nearby star luminosity function. In addition we cross-identified the NLTT and DENIS catalogues to find 15 similar stars, in parts of the sky not yet covered by the colour-selected search.Comment: Accepted by Astronomy & astrophysics. 17 pages, 8 figure

Spectroscopic characterization of 78 DENIS ultracool dwarf candidates in the solar neighborhood and the Upper Sco OB association

Aims: Low-resolution optical spectroscopic observations for 78 very low-mass star and brown dwarf candidates that have been photometrically selected using the DENIS survey point source catalogue. Methods: Spectral types are derived for them using measurements of the PC3 index. They range from M6 to L4. H_alpha emission and NaI subordinate doublet (818.3 nm and 819.9 nm) equivalent widths are measured in the spectra. Spectroscopic indices of TiO, VO, CrH and FeH molecular features are also reported. Results: A rule-of-thumb criterion to select young very low-mass objects using the NaI doublet equivalent width is given. It is used to confirm seven new members of the Upper Sco OB association and two new members of the R Cr-A star-forming region. Four of our field objects are also classified as very young, but are not members of any known nearby young association. The frequency of lower-gravity young objects in our field ultracool sample is 8.5%. Our results provide the first spectroscopic classification for 38 ultracool dwarfs in the solar vicinity with spectrophotometric distances in the range 17 pc to 65 pc (3 of them are new L dwarfs within 20 pc).Comment: 25 pages, 10 figures, 7 tables, Accepted by A&

New neighbours VI. Spectroscopy of DENIS nearby stars candidates

We present spectra of 36 nearby star candidates and 3 red giant candidates, identified in the DENIS database by Phan-Bao et al. (2003). 32 of the dwarf candidates are indeed nearby red dwarfs, with spectral types from M5.5 to M8.5. Out of 11 targets with low proper motion (mu < 0.1$ arc-sec/yr) but a Reduced Proper Motion above an inclusive threshold, 9 are red dwarfs. The 4 contaminants are all reddened F-K main sequence stars, and they could have been eliminated by checking for some well known high latitude molecular clouds. These stars might be of interest as probes of interstellar absorption. For the red dwarfs we derive spectral types and spectroscopic distances, using a new calibration of the PC3 spectral index to absolute magnitudes in the I, J, H and K photometric bands. We confirm 2 new members of the 12 pc volume (2 new M8.5), and one M7.5 NLTT object closer than 10pc. We also show that one quarter of the stars with photometric distances under 30 pc have too small a proper motion for inclusion in the NLTT catalog.Comment: Accepted by A&