arXiv subjects

M. F. Sterzik

Publications and source records attributed to M. F. Sterzik.

25 records · Page 2Linked to original sources

arXiv2004

Identification of a complete sample of northern ROSAT All-Sky Survey X-ray sources. VIII. The late-type stellar component

We present results of an investigation of the X-ray properties, age distribution, and kinematical characteristics of a high-galactic latitude sample of late-type field stars selected from the ROSAT All-Sky Survey (RASS). The sample comprises 254 RASS sources with optical counterparts of spectral types F to M distributed over six study areas located at |b| >= 20 degr, and DEC >= -9 degr. A detailed study was carried out for the subsample of ~200 G, K, and M stars. Lithium abundances were determined for 179 G-M stars. Radial velocities were measured for most of the 141 G and K type stars of the sample. Combined with proper motions these data were used to study the age distribution and the kinematical properties of the sample. Based on the lithium abundances half of the G-K stars were found to be younger than the Hyades (660 Myr). About 25% are comparable in age to the Pleiades (100 Myr). A small subsample of 10 stars is younger than the Pleiades. They are therefore most likely pre-main sequence stars. Kinematically the PMS and Pleiades-type stars appear to form a group with space velocities close to the Castor moving group but clearly distinct from the Local Association.

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arXiv2004

Grain growth and dust settling in a brown dwarf disk: Gemini/T-ReCS observations of CFHT-BD-Tau 4

We present accurate mid-infrared observations of the disk around the young, bona-fide brown dwarf CFHT-BD-Tau 4. We report GEMINI/T-ReCS measurements in the 7.9, 10.4 and 12.3 micron filters, from which we infer the presence of a prominent, broad silicate emission feature. The shape of the silicate feature is dominated by emission from 2 micron amorphous olivine grains. Such grains, being an order of magnitude larger than those in the interstellar medium, are a first proof of dust processing and grain growth in disks around brown dwarfs. The object's spectral energy distribution is below the prediction of the classical flared disk model but higher than that of the two-layer flat disk. A good match can be achieved by using an intermediate disk model with strongly reduced but non-zero flaring. Grain growth and dust settling processes provide a natural explanation for this disk geometry and we argue that such intermediate flaring might explain the observations of several other brown dwarf disks as well.

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arXiv2004

Late-type field stars in the RASS at high galactic latitude

We present results of an investigation on a high-galactic latitude sample of late-type field stars selected from the ROSAT All-Sky Survey (RASS). The sample comprises about 200 G, K, and M stars. Lithium abundances were determined for about 180 G-M stars. Radial velocities were measured for most of the circ. 140 G and K type stars. Combined with proper motions these data were used to study the age distribution and the kinematical properties of the sample. Based on the lithium abundances half of the G-K stars were found to be younger than the Hyades (660 Myr). About 30% are comparable in age to the Pleiades (100 Myr). A small subsample of 10 stars is younger than the Pleiades (100 Myr). They are therefore most likely pre-main sequence stars. Kinematically the PMS and Pleiades-type stars appear to form a group with space velocities close to the Castor moving group but clearly distinct from the Local Association.

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arXiv2002

Probing Dust around Brown Dwarfs: The naked LP 944-20 and the Disk of Cha Ha2

We present the first mid-infrared (MIR) detection of a field brown dwarf (BD) and the first ground-based MIR measurements of a disk around a young BD candidate. We prove the absence of warm dust surrounding the field BD LP 944-20. In the case of the young BD candidate Cha Ha2, we find clear evidence for thermal dust emission from a disk. Surprisingly, the object does not exhibit any silicate feature as previously predicted. We show that the flat spectrum can be explained by an optically thick flat dust disk.

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arXiv1999

The late-type stellar component in the ROSAT All-Sky Survey at high galactic latitude

We investigated the properties of the late-type stellar component in the RASS at high-galactic latitude |b| based on an optically identified sample of ROSAT All-Sky Survey (RASS) X-ray sources. The stellar sample comprises ~250 objects in six study areas covering 685 deg^2 at |b|> 20 deg. We spectroscopically detected a significant fraction of lithium-rich pre-main sequence (PMS) objects including even M-type stars. In an area located about 20 deg south of the Tau-Aur star formation region (SFR) and close to the Gould Belt, we found a large fraction of 40% PMS stars among the K-type stellar counterparts. In other areas we found a smaller but still significant fraction of Li-rich stars. We compared the log N - log S distribution with published galactic distribution models for different age groups and with results from the ROSAT Galactic Plane Survey. For the sample south of Tau-Aur we find an excess of PMS stars compared to model calculations while in the other areas the observed log N - log S is close to the model predictions. We started to investigate the proper motions and radial velocities of both, the young lithium-rich and the older stellar counterparts. Radial velocities and proper motions of the Li-rich stars in the area south of Tau-Aur are consistent with membership to the Tau-Aur SFR. The non-PMS stars show a wider spread in radial velocities and proper motions.

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arXiv1997

New proper motions of pre-main sequence stars in Taurus-Auriga

We present proper motions of 72 T Tauri stars located in the central region of Taurus-Auriga. These proper motions are taken from a new proper motion catalogue called STARNET. Our sample comprises 17 classical T Tauri stars and 55 weak-line T Tauri stars, most of the latter discovered by ROSAT. 53 stars had no proper motion measurement before. Kinematically, 62 of these stars are members of the association. A velocity dispersion of less than 2-3 km/s is found which is dominated by the errors of the proper motions. This velocity dispersion correlates with a spread in distances. Furthermore we present proper motions of 58 stars located in a region just south of the Taurus molecular clouds and compare the kinematics of the youngest stars in this sample (younger than 3.5 10^7 yrs) with the kinematics of the pre-main sequence stars in the Taurus-Auriga association. From a comparison of the space velocities we find that the stars in the central region of Tau-Aur are kinematically different from the stars in the southern part. Among the stars with large proper motions far off the Taurus mean motion we find 2 Pleiades candidates and 7 possible Pleiades runaway stars.

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arXiv1996

Search for young low-mass stars in a ROSAT selected sample south of the Taurus-Auriga molecular clouds

We present results of intermediate resolution spectroscopy of 131 optical counterparts to 115 ROSAT All-Sky Survey X-ray sources south of the Taurus-Auriga dark cloud complex. These objects have been selected as candidate young stars from a total of 1084 ROSAT sources in a about 300 square degree area. We identify 30 objects as low-mass PMS stars on the basis of the Li 6708 doublet in their spectrum, a signature of their young age. All these stars have a spectral type later than F7 and show spectral characteristics typical of weak-line and post-T Tauri stars. The presence of young objects several parsecs away from the regions of ongoing star formation is discussed in the light of the current models of T Tauri dispersal.

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