H. Jahreiß

Towards a fully consistent Milky Way disc model – I. 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 subpopulations described by the star formation history (SFR) and the dynamical heating law [given by the age-velocity dispersion relation (AVR)]. The stellar subpopulations are in dynamical equilibrium and the gravitational potential is calculated self-consistently including the influence of the dark matter halo and the gas component. 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 initial mass function (IMF). The disc parameters are determined by applying a sophisticated best-fitting 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. The model is consistent with the local kinematics of MS stars and fulfils the known constraints on scaleheights, surface densities and mass ratios. 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 and 25.0 km s -1 , 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 ⊙ pc -2 Gyr -1 . A best fit of the IMF leads to power-law indices of -1.46 below and -4.16 above 1.72M ⊙ avoiding a kink at 1 M ⊙ . An isothermal thick-disc component with local density of ~6 per cent of the stellar density is included. A thick disc containing more than 10 per cent of local stellar mass is inconsistent with the local kinematics of K and M dwarfs. Neglecting the thick-disc component results in slight variations of the thin-disc properties, but has a negligible influence on the AVR and the normalized SFR. The model allows detailed predictions of the density, age, metallicity and velocity distribution functions of MS stars as a function of height above the mid-plane. The complexity of the model does not allow to rule out other star formation scenarios using the local data alone. The incorporation of multiband star count and kinematic data of larger samples in the near future will improve the determination of the disc structure and evolution significantly.

Search for nearby stars among proper motion stars selected by optical-to-infrared photometry III. Spectroscopic distances of 322 NLTT stars ⋆,⋆⋆

Distance estimates based on low-resolution spectroscopy and Two Micron All Sky Survey (2MASS) J magnitudes are presented for a large sample of 322 nearby candidates from Luytens NLTT catalogue. Mainly relatively bright (typically 7 100 pc) objects have also been identified, including a probable halo, but relatively hot (T eff ∼ 13 000 K) white dwarf (LHS 1200) and 10 red dwarfs with extremely large tangential velocities (250 < v t < 1150 km s -1 ). Altogether, there are 11 red dwarfs (including one within 70 pc) with tangential velocities larger than about 250 km s -1 . All these objects are suspected to be in fact subdwarfs, if so, their distances would he only about half of our original estimates. The three most extreme objects in that respect are the K and early M dwarfs LP 323-168, LHS 5343 and LP 552-21 with corrected distances between 180 pc and 400 pc and resulting tangential velocities still larger than about 400 km s -1 .

A Schmidt-Kennicutt law for star formation in the Milky Way disk

We use a new method to trace backward the star formation history of the Milky Way disk, using a sample of M dwarfs in the solar neighborhood that is representative for the entire solar circle. M stars are used because they show Hα 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 disks lifetime. Our star formation rate (SFR) agrees well with those obtained by using other independent methods and seems to rule out a constant SFR. The principal result of this study is to show that a relation of the Schmidt-Kennicut type (which relates the SFR to the interstellar gas content of galaxy disks) has pertained in the Milky Way disk during the last 5 Gyr. The SFR 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 SFR and gas surface densities over the last 5 Gyr can be accurately described by a Schmidt-Kennicutt law with an index of Γ = 1.45+0.22 –0.09. This is, within statistical uncertainties, the same value found for other galaxies.

Nearby star candidates in the Torino observatory parallax program

Aims. Candidates with suspect distances from the Catalog of Nearby Stars were included in the Torino Observatory Parallax Program with the goal to clarify their membership in that catalog. Methods. Observations of the objects were made over the period 1996-2001 on the 1.05 m Torino telescope. The trigonometric parallaxes and proper motions were determined using standard techniques. Results. Of the 22 objects examined, 11 are within the 25 pc horizon of the Catalog of Nearby Stars. The remaining objects are either misclassified, sub-dwarfs rather than main sequence dwarfs with consequently overestimated photometric distances, or objects with published trigonometric parallaxes that are incorrect.

Search for nearby stars among proper motion stars selected by optical-to-infrared photometry - I. Discovery of LHS 2090 at spectroscopic distance of

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

\mathsf{{\vec d} \sim 6}

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

pc

R-K_s

Spectroscopic distance estimates for fourteen faint red LHS and NLTT stars

colour index and with relatively bright infrared magnitudes (

The local stellar luminosity function and mass-to-light ratio in the near-infrared

K_s<10

Spectroscopic distances of 28 nearby star candidates

) 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