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Register a new userMeeting the Cool Neighbors VII: Spectroscopy of faint, red NLTT dwarfs
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We present low-resolution optical spectroscopy and BVRI photometry of 453 candidate nearby stars drawn from the NLTT proper motion catalogue. The stars were selected based on optical/near-infrared colours, derived by combining the NLTT photographic data with photometry from the 2MASS Second Incremental Data Release. Based on the derived photometric and spectroscopic parallaxes, we identify 111 stars as lying within 20 parsecs of the Sun, including 9 stars with formal distance estimates of less than 10 parsecs. A further 53 stars have distance estimates within 1-sigma of our 20-parsec limit. Almost all of those stars are additions to the nearby star census. In total, our NLTT-based survey has so far identified 496 stars likely to be within 20 parsecs, of which 195 are additions to nearby-star catalogues. Most of the newly-identified nearby stars have spectral types between M4 and M8.
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Continuing our census of late-type dwarfs in the Solar Neighbourhood, we present BVRI photometry and optical spectroscopy of 800 mid-type M dwarfs drawn from the NLTT proper motion catalogue. The targets are taken from both our own cross-referencing of the NLTT catalogue and the 2MASS Second Incremental release, and from the revised NLTT compiled by Salim & Gould (2003). All are identified as nearby-star candidates based on their location in the (m_r, (m_r-K_S)) diagram. Three hundred stars discussed here have previous astrometric, photometric or spectroscopic observations. We present new BVRI photometry for 101 stars, together with low resolution spectroscopy of a further 400 dwarfs. In total, we find that 241 stars are within 20 parsecs of the Sun, while a further 70 lie within 1-sigma of our distance limit. Combining the present results with previous analyses, we have quantitative observations for 1910 of the 1913 candidates in our NLTT nearby-star samples. Eight hundred and fifteen of those stars have distance estimates of 20 parsecs or less, including 312 additions to the local census. With our NLTT follow-up observations essentially complete, we have searched the literature for K and early-type M dwarfs within the sampling volume covered by the 2MASS Second Release. Comparing the resultant 20-parsec census against predicted numbers, derived from the 8-parsec luminosity function, shows an overall deficit of ~20% for stellar systems and ~35% for individual stars. Almost all are likely to be fainter than M_J=7, and at least half are probably companions of known nearby stars. Our results suggest that there are relatively few missing systems at the lowest luminosities, M_J > 8.5. We discuss possible means of identifying the missing stars.
Using data from the 2MASS All-Sky Point Source Catalogue, we have extended our census of nearby ultracool dwarfs to cover the full celestial sphere above Galactic latitute 15 degrees. Starting with an initial catalogue of 2,139,484 sources, we have winnowed the sample to 467 candidate late-type M or L dwarfs within 20 parsecs of the Sun. Fifty-four of those sources already have spectroscopic observations confirming them as late-type dwarfs. We present optical spectroscopy of 376 of the remaining 413 sources, and identify 44 as ultracool dwarfs with spectroscopic distances less than 20 parsecs. Twenty-five of the 37 sources that lack optical data have near-infrared spectroscopy. Combining the present sample with our previous results and data from the literature, we catalogue 94 L dwarf systems within 20 parsecs. We discuss the distribution of activity, as measured by H-alpha emission, in this volume-limited sample. We have coupled the present ultracool catalogue with data for stars in the northern 8-parsec sample and recent (incomplete) statistics for T dwarfs to provide a snapshot of the current 20-parsec census as a function of spectral type.
Asteroseismology has proven to be an excellent tool to determine not only the global stellar properties with a good precision but also to infer stellar structure, dynamics, and evolution for a large sample of Kepler stars. Prior to the launch of the mission the properties of Kepler targets were inferred from broadband photometry, leading to the Input Catalog (KIC Brown et al. 2011). The KIC was later revised in the Kepler Star Properties Catalog (Huber et al. 2014), based on literature values and an asteroseismic analysis of stars which were unclassified in the KIC. Here we present an asteroseismic analysis of 45,400 stars which were classified as dwarfs in the Kepler Star Properties Catalog. We found that around 2% of the sample shows acoustic modes in the typical frequency range that put them in the red-giant category rather than cool dwarfs. We analyse the asteroseismic properties of these stars, derive their surface gravities, masses, and radii and present updated effective temperatures and distances. We show that the sample is significantly fainter than the previously known oscillating giants in the Kepler field, with the faintest stars reaching down to a Kepler magnitude, Kp~16. We demonstrate that 404 stars are at distances beyond 5 kpc and that the stars are significantly less massive than for the original Kepler red-giant sample, consistent with a population of distant halo giants. A comparison with a galactic population model shows that up to 40 stars might be genuine halo giants, which would increase the number of known asteroseismic halo stars by a factor of 4. The detections presented here will provide a valuable sample for galactic archeology studies.
We present precise photometry and spectroscopy for 23 candidate spectrophotometric standard white dwarfs. The selected stars are distributed in the Northern hemisphere and around the celestial equators and are all fainter than r ~ 16.5 mag. This network of stars, when established as standards, together with the three Hubble Space Telescope primary CALSPEC white dwarfs, will provide a set of spectrophotometric standards to directly calibrate data products to better than 1%. These new faint standard white dwarfs will have enough signal-to-noise ratio in future deep photometric surveys and facilities to be measured accurately while still avoiding saturation in such surveys. They will also fall within the dynamic range of large telescopes and their instruments for the foreseeable future. This paper discusses the provenance of the observational data for our candidate standard stars. The comparison with models, reconciliation with reddening, and the consequent derivation of the full spectral energy density distributions for each of them is reserved for a subsequent paper.
We discuss three topics related to the neutron star (NS) mass spectrum. At first we discuss the possibility to form low-mass ($ M stackrel{<}{sim} 1 M_{odot}$) and suggest this is possible only due to fragmentation of rapidly rotating proto-NSs. Such low-mass NSs should have very high spatial velocities which could allow identification. A critical assessment of this scenario is given. Secondly, we discuss mass growth due to accretion for NSs in close binary systems. With the help of numerical population synthesis calculations we derive the mass spectrum of massive ($M > 1.8 M_{odot}$) NSs. Finally, we discuss the role of the mass spectrum in population studies of young cooling NSs. We formulate a kind of {it mass constraint} which can be helpful, in our opinion, in discussing different competive models of the thermal evolution of NSs.
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