118 research outputs found
A Schmidt-Kennicutt law for star formation in the Milky Way disk
We use a new method to trace backwards the star formation history of the
Milky Way disk, using a sample of M dwarfs in the solar neighbourhood which is
representative for the entire solar circle. M stars are used because they show
H_alpha emission until a particular age which is a well calibrated function of
their absolute magnitudes. This allows us to reconstruct the rate at which disk
stars have been born over about half the disk's lifetime. Our star formation
rate agrees well with those obtained by using other, independent, methods and
seems to rule out a constant star formation rate.
The principal result of this study is to show that a relation of the
Schmidt-Kennicut type (which relates the star formation rate to the
interstellar gas content of galaxy disks) has pertained in the Milky Way disk
during the last 5 Gyr. The star formation rate we derive from the M dwarfs and
the interstellar gas content of the disk can be inferred as a function of time
from a model of the chemical enrichment of the disk, which is well constrained
by the observations indicating that the metallicity of the Galactic disk has
remained nearly constant over the timescales involved. We demonstrate that the
star formation rate and gas surface densities over the last 5 Gyrs can be
accurately described by a Schmidt-Kennicutt law with an index of Gamma = 1.45
(+0.22,-0.09). This is, within statistical uncertainties, the same value found
for other galaxies.Comment: 7 pages, 5 figures, accepted by Astron.
New nearby white dwarfs from Gaia DR1 TGAS and UCAC5/URAT
Using an accurate Gaia TGAS 25pc sample, nearly complete for GK stars, and
selecting common proper motion (CPM) candidates from UCAC5, we search for new
white dwarf (WD) companions around nearby stars with relatively small proper
motions. For investigating known CPM systems in TGAS and for selecting CPM
candidates in TGAS+UCAC5, we took into account the expected effect of orbital
motion on the proper motion as well as the proper motion catalogue errors.
Colour-magnitude diagrams (CMDs) and were used to verify
CPM candidates from UCAC5. Assuming their common distance with a given TGAS
star, we searched for candidates that occupied similar regions in the CMDs as
the few known nearby WDs (4 in TGAS) and WD companions (3 in TGAS+UCAC5). CPM
candidates with colours and absolute magnitudes corresponding neither to the
main sequence nor to the WD sequence were considered as doubtful or subdwarf
candidates. With a minimum proper motion of 60mas/yr, we selected three WD
companion candidates, two of which are also confirmed by their significant
parallaxes measured in URAT data, whereas the third may also be a chance
alignment of a distant halo star with a nearby TGAS star (angular separation of
about 465arcsec). One additional nearby WD candidate was found from its URAT
parallax and photometry. With HD 166435 B orbiting a well-known G1 star
at ~24.6pc with a projected physical separation of ~700AU, we discovered one of
the hottest WDs, classified by us as DA2.00.2, in the solar neighbourhood.
We also found TYC 3980-1081-1 B, a strong cool WD companion candidate around a
recently identified new solar neighbour with a TGAS parallax corresponding to a
distance of ~8.3pc and our photometric classification as ~M2 dwarf. This raises
the question whether previous assumptions on the completeness of the WD sample
to a distance of 13pc were correct.Comment: 9 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
Search for nearby stars among proper motion stars selected by optical-to-infrared photometry. I. Discovery of LHS 2090 at spectroscopic distance of d=6pc
We present the discovery of a previously unknown very nearby star - LHS 2090
at a distance of only d=6 pc. In order to find nearby (i.e. d < 25 pc) red
dwarfs, we re-identified high proper motion stars ( 0.18 arcsec/yr) from
the NLTT catalogue (Luyten \cite{luyten7980}) in optical Digitized Sky Survey
data for two different epochs and in the 2MASS data base. Only proper motion
stars with large colour index and with relatively bright infrared
magnitudes () were selected for follow-up spectroscopy. The
low-resolution spectrum of LHS 2090 and its large proper motion (0.79
arcsec/yr) classify this star as an M6.5 dwarf. The resulting spectroscopic
distance estimate from comparing the infrared magnitudes of LHS 2090
with absolute magnitudes of M6.5 dwarfs is pc assuming an
uncertainty in absolute magnitude of 0.4 mag.Comment: 3 pages, 1 figure, accepted for publication in Astronomy and
Astrophysics Letter
Spectroscopic classification and Gaia DR2 parallaxes of new nearby white dwarfs among selected blue proper motion stars
AIMS: With our low-resolution spectroscopic observing program for selected
blue proper motion stars, we tried to find new white dwarfs (WDs) in the solar
neighbourhood. METHODS: We used the LSPM catalogue with a lower proper motion
limit of 150mas/yr and the UCAC2 for proper motions down to about 90mas/yr. The
LSPM and UCAC2 photometry was combined with Two Micron All Sky Survey (2MASS)
near-infrared (NIR) photometry. Targets selected according to their blue
optical-to-NIR and NIR colours were observed mainly at Calar Alto. The spectra
were classified by comparison with a large number of already known comparison
objects, including WDs, simultaneously observed within our program. Gaia DR2
parallaxes and colours were used to confirm or reject spectroscopic WD
candidates and to derive improved effective temperatures. RESULTS: We found ten
new WDs at distances between 24.4pc and 79.8pc, including six hot DA WDs: GD
221 (DA2.0), HD 166435 B (DA2.2), GD 277 (DA2.2), 2MASS J19293865+1117523
(DA2.4), 2MASS J05280449+4105253 (DA3.6), and 2MASS J05005185-0930549 (DA4.2).
The latter is rather bright (G~12.6) and with its Gaia DR2 parallax of ~14mas
it appears overluminous by about 3mag compared to the WD sequence in the Gaia
DR2 colour-magnitude diagram. It may be the closest extremely low mass (ELM) WD
to the Sun. We further classified 2MASS J07035743+2534184 as DB4.1. With its
distance of 25.6pc it is the second nearest known representative of its class.
With GD 28 (DA6.1), LP 740-47 (DA7.5), and LSPM J1919+4527 (DC10.3) three
additional cool WDs were found. Gaia DR2 parallaxes showed us that four of our
candidates but also two previously supposed WDs (WD 1004+665 and LSPM
J1445+2527) are in fact distant Galactic halo stars with high tangential
velocities. Among our rejected WD candidates, we identified a bright
(G=13.4mag) G-type carbon dwarf, LSPM J0937+2803, at a distance of 272pc.Comment: 14 pages, 13 figures, accepted for publication in Astronomy and
Astrophysic
Kinematics of Nearby Subdwarf Stars
We present an analysis of the space motions of 742 subdwarf stars based on
the sample of Carney et al. (1994, CLLA). Hipparcos parallaxes, TYC2+HIP proper
motions and Tycho2 proper motions were combined with radial velocities and
metallicities from CLLA. The kinematical behavior is discussed in particular in
relation to their metallicities. The majority of these sample stars have metal
abundances of [Fe/H] >-1 and represent the thick disk population. The halo
component, with [Fe/H] <-1.6, is characterized by a low mean rotation velocity
and a radially elongated velocity ellipsoid. In the intermediate metallicity
range (-1.6 < [Fe/H] <-1), we find a significant number of subdwarfs with
disklike kinematics. We interpret this population of stars as a metal-weak
thick disk population.Comment: 6 pages, 7 figures, accepted by Astronomy & Astrophysic
The kinematics of late type stars in the solar cylinder studied with SDSS data
We study the velocity distribution of Milky Way disk stars in a
kiloparsec-sized region around the Sun, based on ~ 2 million M-type stars from
DR7 of SDSS, which have newly re-calibrated absolute proper motions from
combining SDSS positions with the USNO-B catalogue. We estimate photometric
distances to all stars, accurate to ~ 20 %, and combine them with the proper
motions to derive tangential velocities for this kinematically unbiased sample
of stars. Based on a statistical de-projection method we then derive the
vertical profiles (to heights of Z = 800 pc above the disk plane) for the first
and second moments of the three dimensional stellar velocity distribution. We
find that = -7 +/- 1 km/s and = -9 +/- 1 km/s, independent of height
above the mid-plane, reflecting the Sun's motion with respect to the local
standard of rest. In contrast, changes distinctly from -20 +/- 2 km/s in
the mid-plane to = -32 km/s at Z = 800 pc, reflecting an asymmetric drift
of the stellar mean velocity that increases with height. All three components
of the M-star velocity dispersion show a strong linear rise away from the
mid-plane, most notably \sigma_{ZZ}, which grows from 18 km/s (Z = 0) to 40
km/s (at Z = 800 pc). We determine the orientation of the velocity ellipsoid,
and find a significant vertex deviation of 20 to 25 degrees, which decreases
only slightly to heights of Z = 800 pc. Away from the mid-plane, our sample
exhibits a remarkably large tilt of the velocity ellipsoid towards the Galactic
plane, which reaches 20 deg. at Z = 800 pc and which is not easily explained.
Finally, we determine the ratio \sigma^2_{\phi\phi}/\sigma^2_{RR} near the
mid-plane, which in the epicyclic approximation implies an almost perfectly
flat rotation curve at the Solar radius.Comment: 18 pages, 9 figures, accepted to Astron.
Towards a fully consistent Milky Way disc model - II. The local disc model and SDSS data of the NGP region
We have used the self-consistent vertical disc models of the solar
neighbourhood presented in Just & Jahreiss (2010), which are based on different
star formation histories (SFR) and fit the local kinematics of main sequence
stars equally well, to predict star counts towards the North Galactic Pole
(NGP). We combined these four different models with the local main sequence in
the filter system of the SDSS and predicted the star counts in the NGP field
with b>80deg. All models fit the Hess diagrams in the F-K dwarf regime better
than 20 percent and the star number densities in the solar neighbourhood are
consistent with the observed values. The chi^2 analysis shows that model A is
clearly preferred with systematic deviations of a few percent only. The SFR of
model A is characterised by a maximum at an age of 10Gyr and a decline by a
factor of four to the present day value of 1.4Msun/pc^2/Gyr. The thick disc can
be modelled very well by an old isothermal simple stellar population. The
density profile can be approximated by a sech^(alpha_t) function. We found a
power law index alpha_t=1.16 and a scale height of 800pc corresponding to a
vertical velocity dispersion of 45.3km/s. About 6 percent of the stars in the
solar neighbourhood are thick disc stars.Comment: 11 pages, 12 figures, accepted by MNRAS. The qualtity of figures 10
and 12 are much better in the .ps version than the .pdf versio
Towards a fully consistent Milky Way disc model: Part 1 The local model based on kinematic and photometric data
We present a fully consistent evolutionary disc model of the solar cylinder.
The model is based on a sequence of stellar sub-populations described by the
star formation history (SFR) and the dynamical heating law (given by the
age-velocity dispersion relation AVR). The combination of kinematic data from
Hipparcos and the finite lifetimes of main sequence (MS) stars enables us to
determine the detailed vertical disc structure independent of individual
stellar ages and only weakly dependent on the IMF. The disc parameters are
determined by applying a sophisticated best fit algorithm to the MS star
velocity distribution functions in magnitude bins. We find that the AVR is well
constrained by the local kinematics, whereas for the SFR the allowed range is
larger. A simple chemical enrichment model is included in order to fit the
local metallicity distribution of G dwarfs. In our favoured model A the power
law index of the AVR is 0.375 with a minimum and maximum velocity dispersion of
5.1 km/s and 25.0 km/s, respectively. The SFR shows a maximum 10 Gyr ago and
declines by a factor of four to the present day value of 1.5 M_sun/pc^2/Gyr. A
best fit of the IMF leads to power-law indices of -1.46 below and -4.16 above
1.72 M_sun avoiding a kink at 1 M_sun. An isothermal thick disc component with
local density of ~6% of the stellar density is included. A thick disc
containing more than 10% of local stellar mass is inconsistent with the local
kinematics of K and M dwarfs.Comment: 20 pages, 18 figs., accepted by MNRA
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)Comment: 5 pages, 4 figures, accepted by Astron. Astrophy
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