arXiv subjects

R. -D. Scholz

Publications and source records attributed to R. -D. Scholz.

At least 55 records · Page 3Linked to original sources

arXiv2008

The initial luminosity and mass functions of the Galactic open clusters

(... abridged) The observed luminosity function can be constructed in a range of absolute integrated magnitudes $I_{M_V}= [-10, -0.5]$ mag, i.e. about 5 magnitudes deeper than in the most nearby galaxies. It increases linearly from the brightest limit to a turnover at about $I_{M_V}\approx-2.5$. The slope of this linear portion is $a=0.41\pm0.01$, which agrees perfectly with the slope deduced for star cluster observations in nearby galaxies. (...) We find that the initial mass function of open clusters (CIMF) has a two-segment structure with the slopes $α=1.66\pm0.14$ in the range $\log M_c/M_\odot=3.37...4.93$ and $α=0.82\pm0.14$ in the range $\log M_c/M_\odot=1.7...3.37$. The average mass of open clusters at birth is $4.5\cdot 10^3 M_\odot$, which should be compared to the average observed mass of about $700 M_\odot$. The average cluster formation rate derived from the comparison of initial and observed mass functions is $\bar{\upsilon}=0.4 \mathrm{kpc}^{-2}\mathrm{Myr}^{-1}$. Multiplying by the age of the Galactic disc (T = 13 Gyr) the predicted surface density of Galactic disc field stars originating from dissolved open clusters amounts to $22 M_\odot \mathrm{pc}^{-2}$ which is about 40% of the total surface density of the Galactic disc in the solar neighbourhood. Thus, we conclude that almost half of all field stars were born in open clusters, a much higher fraction than previously thought.

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arXiv2008

PPM-Extended (PPMX) - a catalogue of positions and proper motions

Aims: We build a catalogue PPM-Extended (PPMX) on the ICRS system which is complete down to a well-defined limiting magnitude and contains the best presently available proper motions to be suited for kinematical studies in the Galaxy. Methods: We perform a rigorous weighted least-squares adjustment of individual observations, spread over more than a century, to determine mean positions and proper motions. The stellar content of PPMX is taken from GSC 1.2 supplemented by catalogues like ARIHIP, PPM and Tycho-2 at the bright end. All observations have been weighted according to their individual accuracy. The catalogue has been screened towards rejecting false entries in the various source catalogues. Results: PPM-Extended (PPMX) is a catalogue of 18,088,920 stars containing astrometric and photometric information. Its limiting magnitude is about 15.2 in the GSC photometric system. PPMX consists of three parts: a) a survey complete down to R_U = 12.8 in the magnitude system of UCAC2; b) additional stars of high-precision proper motions, and c) all other stars from GSC 1.2 identified in 2MASS. The typical accuracy of the proper motions is 2mas/y for 66 percent of the survey stars (a) and the high-precision stars (b), and about 10 mas/y for all other stars. PPMX contains photometric information from ASCC-2.5 and 2MASS.

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arXiv2008

The Radial Velocity Experiment (RAVE): second data release

We present the second data release of the Radial Velocity Experiment (RAVE), an ambitious spectroscopic survey to measure radial velocities (RVs) and stellar atmosphere parameters of up to one million stars using the 6dF multi-object spectrograph on the 1.2-m UK Schmidt Telescope of the Anglo-Australian Observatory (AAO). It is obtaining medium resolution spectra (median R=7,500) in the Ca-triplet region (8,410--8,795 Å) for southern hemisphere stars in the magnitude range 9<I<12. Following the first data release (Steinmetz et al. 2006) the current release doubles the sample of published RVs, now containing 51,829 RVs for 49,327 individual stars observed on 141 nights between April 11 2003 and March 31 2005. Comparison with external data sets shows that the new data collected since April 3 2004 show a standard deviation of 1.3 km/s, about twice better than for the first data release. For the first time this data release contains values of stellar parameters from 22,407 spectra of 21,121 individual stars. They were derived by a penalized χ^2 method using an extensive grid of synthetic spectra calculated from the latest version of Kurucz models. From comparison with external data sets, our conservative estimates of errors of the stellar parameters (for a spectrum with S/N=40) are 400 K in temperature, 0.5 dex in gravity, and 0.2 dex in metallicity. We note however that the internal errors estimated from repeat RAVE observations of 822 stars are at least a factor 2 smaller. We demonstrate that the results show no systematic offsets if compared to values derived from photometry or complementary spectroscopic analyses. The data release includes proper motion and photometric measurements. It can be accessed via the RAVE webpage: http://www.rave-survey.org and through CDS.

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arXiv2008

Spectroscopic distances of 28 nearby star candidates

28 hitherto neglected candidates for the Catalogue of Nearby Stars (CNS) were investigated to verify their classification and to improve their distance estimates. All targets had at least a preliminary status of being nearby dwarf stars based on their large proper motions and relatively faint magnitudes. Better photometric and/or spectroscopic distances were required for selecting stars which are worth the effort of trigonometric parallax measurements. Low-resolution spectra were obtained with NASPEC at the Tautenburg 2m telescope and with CAFOS at the Calar Alto 2.2m telescope. The spectral types of M-type stars were determined by direct comparison of the target's spectra with those of comparison stars of known spectral types observed with the same instrument. The classification of earlier types was done based on comparison with published spectral libraries. The majority were classified as M dwarfs including 11 stars within 25 pc. The fainter component of LDS 1365, previously thought to form a nearby common proper motion pair, is according to our results an unrelated high-velocity background star. For several other nearby common proper motion pairs our distance estimates of the fainter components are in good agreement with Hipparcos distances of the brighter components. (abridged)

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arXiv2007

The RAVE Survey: Constraining the Local Galactic Escape Speed

We report new constraints on the local escape speed of our Galaxy. Our analysis is based on a sample of high velocity stars from the RAVE survey and two previously published datasets. We use cosmological simulations of disk galaxy formation to motivate our assumptions on the shape of the velocity distribution, allowing for a significantly more precise measurement of the escape velocity compared to previous studies. We find that the escape velocity lies within the range $498\kms < \ve < 608 \kms$ (90 per cent confidence), with a median likelihood of $544\kms$. The fact that $\ve^2$ is significantly greater than $2\vc^2$ (where $\vc=220\kms$ is the local circular velocity) implies that there must be a significant amount of mass exterior to the Solar circle, i.e. this convincingly demonstrates the presence of a dark halo in the Galaxy. For a simple isothermal halo, one can calculate that the minimum radial extent is $\sim58$ kpc. We use our constraints on $\ve$ to determine the mass of the Milky Way halo for three halo profiles. For example, an adiabatically contracted NFW halo model results in a virial mass of $1.42^{+1.14}_{-0.54}\times10^{12}M_\odot$ and virial radius of $305^{+66}_{-45}$ kpc (90 per cent confidence). For this model the circular velocity at the virial radius is $142^{+31}_{-21}\kms$. Although our halo masses are model dependent, we find that they are in good agreement with each other.

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arXiv2007

Astrophysical supplements to the ASCC-2.5. Ia. Radial velocities of about 55000 stars and mean radial velocities of 516 Galactic open clusters and associations

We present the 2nd version of the Catalogue of Radial Velocities with Astrometric Data (CRVAD-2). This is the result of the cross-identification of stars from the All-Sky Compiled Catalogue of 2.5 Million Stars (ASCC-2.5) with the General Catalogue of Radial Velocities and with other recently published radial velocity lists and catalogues. The CRVAD-2 includes accurate J2000 equatorial coordinates, proper motions and trigonometric parallaxes in the Hipparcos system, $B, V$ photometry in the Johnson system, spectral types, radial velocities (RVs), multiplicity and variability flags for 54907 ASCC-2.5 stars. We have used the CRVAD-2 for a new determination of mean RVs of 363 open clusters and stellar associations considering their established members from proper motions and photometry in the ASCC-2.5. For 330 clusters and associations we compiled previously published mean RVs from the literature, critically reviewed and partly revised them. The resulting Catalogue of Radial Velocities of Open Clusters and Associations (CRVOCA) contains about 460 open clusters and about 60 stellar associations in the Solar neighbourhood. These numbers still represent less than 30% of the total number of about 1820 objects currently known in the Galaxy. The mean RVs of young clusters are generally better known than those of older ones.

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arXiv2007

Towards absolute scales of radii and masses of open clusters

Aims: In this paper we derive tidal radii and masses of open clusters in the nearest kiloparsecs around the Sun. Methods: For each cluster, the mass is estimated from tidal radii determined from a fitting of three-parametric King's profiles to the observed integrated density distribution. Different samples of members are investigated. Results: For 236 open clusters, all contained in the catalogue ASCC-2.5, we obtain core and tidal radii, as well as tidal masses. The distributions of the core and tidal radii peak at about 1.5 pc and 7 - 10 pc, respectively. A typical relative error of the core radius lies between 15% and 50%, whereas, for the majority of clusters, the tidal radius was determined with a relative accuracy better than 20%. Most of the clusters have tidal masses between 50 and 1000 $m_\odot$, and for about half of the clusters, the masses were obtained with a relative error better than 50%.

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arXiv2006

Optical spectroscopy of high proper motion stars: new M dwarfs within 10 pc and the closest pair of subdwarfs

We present spectra of 59 nearby stars candidates, M dwarfs and white dwarfs, previously identified using high proper motion catalogues and the DENIS database. We review the existing spectral classification schemes and spectroscopic parallax calibrations in the near-infrared $J$-band and derive spectral types and distances of the nearby candidates. 42 stars have spectroscopic distances smaller than 25 pc, three of them being white dwarfs. Two targets lie within 10 pc, one M8 star at 10.0 pc (APMPM J0103-3738), and one M4 star at 8.3 pc (LP 225-57). One star, LHS 73, is found to be among the few subdwarfs lying within 20 pc. Furthermore, together with LHS 72, it probably belongs to the closest pair of subdwarfs we know.

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arXiv2006

Population analysis of open clusters: radii and mass segregation

Aims: Based on our well-determined sample of open clusters in the all-sky catalogue ASCC-2.5 we derive new linear sizes of some 600 clusters, and investigate the effect of mass segregation of stars in open clusters. Methods: Using statistical methods, we study the distribution of linear sizes as a function of spatial position and cluster age. We also examine statistically the distribution of stars of different masses within clusters as a function of the cluster age. Results: No significant dependence of the cluster size on location in the Galaxy is detected for younger clusters (< 200 Myr), whereas older clusters inside the solar orbit turned out to be, on average, smaller than outside. Also, small old clusters are preferentially found close to the Galactic plane, whereas larger ones more frequently live farther away from the plane and at larger Galactocentric distances. For clusters with (V - M_V) < 10.5, a clear dependence of the apparent radius on age has been detected: the cluster radii decrease by a factor of about 2 from an age of 10 Myr to an age of 1 Gyr. A detailed analysis shows that this observed effect can be explained by mass segregation and does not necessarily reflect a real decrease of cluster radii. We found evidence for the latter for the majority of clusters older than 30 Myr. Among the youngest clusters (between 5 and 30 Myr), there are some clusters with a significant grade of mass segregation, whereas some others show no segregation at all. At a cluster age between 50 and 100 Myrs, the distribution of stars of different masses becomes more regular over cluster area. In older clusters the evolution of the massive stars is the most prominent effect we observe.

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arXiv2006

The Radial Velocity Experiment (RAVE): first data release

We present the first data release of the Radial Velocity Experiment (RAVE), an ambitious spectroscopic survey to measure radial velocities and stellar atmosphere parameters (temperature, metallicity, surface gravity) of up to one million stars using the 6dF multi-object spectrograph on the 1.2-m UK Schmidt Telescope of the Anglo-Australian Observatory (AAO). The RAVE program started in 2003, obtaining medium resolution spectra (median R=7,500) in the Ca-triplet region ($λλ$ 8,410--8,795 Å) for southern hemisphere stars drawn from the Tycho-2 and SuperCOSMOS catalogs, in the magnitude range 9<I<12. The first data release is described in this paper and contains radial velocities for 24,748 individual stars (25,274 measurements when including re-observations). Those data were obtained on 67 nights between 11 April 2003 to 03 April 2004. The total sky coverage within this data release is $\sim$4,760 square degrees. The average signal to noise ratio of the observed spectra is 29.5, and 80% of the radial velocities have uncertainties better than 3.4 km/s. Combining internal errors and zero-point errors, the mode is found to be 2 km/s. Repeat observations are used to assess the stability of our radial velocity solution, resulting in a variance of 2.8 km/s. We demonstrate that the radial velocities derived for the first data set do not show any systematic trend with color or signal to noise. The RAVE radial velocities are complemented in the data release with proper motions from Starnet 2.0, Tycho-2 and SuperCOSMOS, in addition to photometric data from the major optical and infrared catalogs (Tycho-2, USNO-B, DENIS and 2MASS). The data release can be accessed via the RAVE webpage: http://www.rave-survey.org.

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arXiv2005

Revisiting the population of Galactic open clusters

We present results of a study of the galactic open cluster population based on the all-sky catalogue ASCC-2.5 (I/280A) compiled from Tycho-2, Hipparcos and other catalogues. The sample of optical clusters from ASCC-2.5 is complete up to about 850 pc from the Sun. The symmetry plane of the clusters' distribution is determined to be at $Z_0=-22\pm4$ pc, and the scale height of open clusters is only $56\pm3$ pc. The total surface density and volume density in the symmetry plane are $Σ=$ 114 kpc$^{-2}$ and $D(Z_0)=1015$ kpc$^{-3}$, respectively. We find the total number of open clusters in the Galactic disk to be of order of 10$^5$ at present. Fluctuations in the spatial and velocity distributions are attributed to the existence of four open cluster complexes (OCCs) of different ages containing up to a few tens of clusters. Members in an OCC show the same kinematic behaviour, and a narrow age spread. We find, that the youngest cluster complex, OCC~1 ($\log t<7.9$), with 19 deg inclination to the Galactic plane, is apparently a signature of Gould's Belt. The most abundant OCC~2 complex has moderate age ($\log t\approx8.45$). The clusters of the Perseus-Auriga group, having the same age as OCC~2, but different kinematics are seen in breaks between Perseus-Auriga clouds. The oldest ($\log t\approx8.85$) and sparsest group was identified due to a large motion in the Galactic anticentre direction. Formation rate and lifetime of open clusters are found to be $0.23\pm0.03$ kpc$^{-2}$Myr$^{-1}$ and 322$\pm$31 Myr, respectively. This implies a total number of cluster generations in the history of the Galaxy between 30 to 40. We estimate that less than about 10% of the total Galactic stellar disk population has ever passed an open cluster membership.

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arXiv2005

Spectroscopic classification of red high proper motion objects in the Southern Sky

We present the results of spectroscopic follow-up observations for a sample of 71 red objects with high proper motions in the range 0.08-1.14 arcsec/yr as detected using APM and SSS measurements of multi-epoch photographic Schmidt plates. Red objects were selected by combining the photographic BjRI magnitudes with 2MASS near-infrared JHKs magnitudes. Some 50 of the 71 spectroscopically classified objects turn out to be late-type (>M6) dwarfs and in more detail, the sample includes 35 ultracool dwarfs with spectral types between M8 and L2, some previously reported, as well as five M-type subdwarfs, including a cool esdM6 object, SSSPM J0500-5406. Distance estimates based on the spectral types and 2MASS J magnitudes place almost all of the late-type (>M6) dwarfs within 50 pc, with 25 objects located inside the 25 pc limit of the catalogue of nearby stars. Most of the early-type M dwarfs are located at larger distances of 100-200 pc, suggesting halo kinematics for some of them. All objects with Halpha equivalent widths larger than 10 Angstroms have relatively small tangential velocities (<50 km/s). Finally, some late-type but blue objects are candidate binaries.

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arXiv2005

Three active M dwarfs within 8 pc: L 449-1, L 43-72, & LP 949-15

Three nearby star candidates were selected as bright 2MASS point sources without associated optical sources, i.e. with potentially large proper motions, subsequently confirmed by multi-epoch optical data from the SuperCOSMOS Sky Surveys. All three objects are listed in the NLTT catalogue of high proper motion stars. Follow-up spectroscopic observations allowed us to classify L 449-1 as M4.0e dwarf at 5.7 pc, L43-72 as M4.5e dwarf at 7.2 pc, and LP 949-15 as M5.0e dwarf at 6.1 pc, using known absolute $JHK_\mathrm{s}$ magnitudes of dwarfs with corresponding spectral types, respectively. All three stars exhibit $Hα$ emission lines, and all three can be identified with bright X-ray sources. The available ROSAT light curves of two of the objects show typical flare-like variability. Thus, all three stars are active, very close potential neighbours of the Sun, previously not mentioned as such, certainly deserving further attention. In particular, these are very promising candidates for trigonometric parallax programs.

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arXiv2005

109 new Galactic open clusters

We present a list of 130 Galactic Open Clusters, found in the All-Sky Compiled Catalogue of 2.5 Million Stars (ASCC-2.5). For these clusters we determined and publish a homogeneous set of astrophysical parameters such as size, membership, motion, distance and age. In a previous work 520 already known open clusters out of the sample of 1700 clusters from the literature were confirmed in the ASCC-2.5 using independent, objective methods. Using these methods the whole sky was systematically screened for the search of new clusters. The newly detected clusters show the same distribution over the sky as the known ones. It is found, that without the {\em a-priori} knowledge about existing clusters our search lead to clusters which are, {\em on average}, brighter, have more members and cover larger angular radii than the 520 previously known ones.

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arXiv2005

VPMS J1342+2840 - an unusual quasar from the variability and proper motion survey

We report the discovery of the highly peculiar, radio-loud quasar VPMS J1342+2840 (z ~ 1.3) from the variability and proper motion survey. We present spectroscopic, imaging and photometric observations. The unusual spectrum shows a strong depression of the continuum over a wide wavelength range in the blue part without the typical structures of broad absorption line (BAL) troughs. The image of the quasar is unresolved and there is no evidence for a foreground object on the line of sight. The broad-band spectral energy distribution is not consistent with obvious dust reddening with the standard SMC extinction curve. The downturn of the continuum flux of VPMS J1342+2840 at short wavelengths can be caused by dust reddening only if the reddening curve is steeper then the SMC curve in the ultraviolet and is very flat at longer wavelengths. Alternatively, the dominant spectral features can be explained by low-ionization BALs forming unusually wide, overlapping absorption troughs.

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arXiv2005

Astrophysical parameters of Galactic open clusters

We present a catalogue of astrophysical data of 520 Galactic open clusters. These are the clusters, for which at least three most probable members (18 on average) could be identified in the ASCC-2.5, a catalogue of stars based on the Tycho-2 observations from the Hipparcos mission. We applied homogeneous methods and algorithms to determine angular sizes of cluster cores and coronae, heliocentric distances, mean proper motions, mean radial velocities and ages. For the first time we derive distances for 200 clusters, radial velocities for 94 clusters and determine ages of 196 clusters. This homogeneous new parameter set is compared with earlier determinations. In particular, we find that the angular sizes were systematically underestimated in the literature.

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arXiv2004

SSSPM J1444-2019: an extremely high proper motion, ultracool subdwarf

We present the discovery of a new extreme high proper motion object (3.5 arcsec/year) which we classify as an ultracool subdwarf with [M/H] = -0.5. It has a formal spectral type of sdM9 but also shows L-type features: while the VO bands are completely absent, it exhibits extremely strong TiO absorption in its optical spectrum. With a radial velocity of about -160 km/s and a rough distance estimate of 16--24 pc, it is likely one of the nearest halo members crossing the Solar neighbourhood with a heliocentric space velocity of (U,V,W)=(-244,-256,-100)+/-(32,77,6) km/s.

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arXiv2004

The nearest cool white dwarf (d~4pc), the coolest M-type subdwarf (sdM9.5), and other high proper motion discoveries

We report the discovery of seven high proper motion stars with proper motions between about 0.7 and 2.2 arcsec/yr. They were detected in an archival search using multi-epoch positions in the optical SuperCOSMOS Sky Surveys and in the near-infrared sky surveys 2MASS and DENIS. The star with the largest proper motion, is spectroscopically classified as a very nearby (d=8pc) M5.5 dwarf with Galactic thin disk kinematics. A second star with about 2 arcsec/yr proper motion, is still lacking spectroscopic confirmation but can be classified from photometry as thick disk M3.5 dwarf. Three objects turn out to be cool subdwarf members of the Galactic thick disk or halo, including the first sdM9.5 object, which represents the currently coolest known M subdwarf, another ultra-cool subdwarf, of spectral type sdM7.0 as well as an earlier type (sdM1.5) star. The latter has an extremely large space velocity with clear halo kinematics. Two objects show featureless spectra typical of cool white dwarfs. One of them is probably a very nearby (d~4pc) thin disk object and may be the nearest isolated white dwarf, the other one has thick disk kinematics.

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