6,498 research outputs found
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
A case study using ECHO(Extraction and Classification of Homogeneous Objects) for analysis of multispectral scanner data
There are no author-identified significant results in this report
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.Comment: accepted on 06 June 2005 for publication in A&A, 22 pages, 14
figures, 7 table
Why Simple Stellar Population models do not reproduce the colours of Galactic open clusters
(...) We search for an explanation of the disagreement between the observed
integrated colours of 650 local Galactic clusters and the theoretical colours
of present-day SSP models. We check the hypothesis that the systematic offsets
between observed and theoretical colours, which are and
, are caused by neglecting the discrete nature of the
underlying mass function. Using Monte Carlo simulations, we construct
artificial clusters of coeval stars taken from a mass distribution defined by
an Salpeter initial mass function (IMF) and compare them with corresponding
"continuous-IMF" SSP models. If the discreteness of the IMF is taken into
account, the model fits the observations perfectly and is able to explain
naturally a number of red "outliers" observed in the empirical colour-age
relation. We find that the \textit{systematic} offset between the continuous-
and discrete-IMF colours reaches its maximum of about 0.5 in for a
cluster mass at ages , and diminishes
substantially but not completely to about one hundredth of a magnitude at at cluster masses . At younger ages, it is still
present even in massive clusters, and for it is
larger than 0.1 mag in . Only for very massive clusters () with ages is the offset small (of the order of 0.04
mag) and smaller than the typical observational error of colours of
extragalactic clusters.Comment: 4 pages, 3 figures, accepted for publication in Astronomy and
Astrophysics Letters, revised version after language editing and with an
additional reference to Cervino and Luridiana (2004
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.Comment: 9 pages, 8 figures, accepted for publication in Astronomy and
Astrophysic
Observations of Mira stars with the IOTA/FLUOR interferometer and comparison with Mira star models
We present K'-band observations of five Mira stars with the IOTA
interferometer. The interferograms were obtained with the FLUOR fiber optics
beam combiner, which provides high-accuracy visibility measurements in spite of
time-variable atmospheric conditions. For the M-type Miras X Oph, R Aql, RU
Her, R Ser, and the C-type Mira V CrB we derived the uniform-disk diameters
11.7mas, 10.9mas, 8.4mas, 8.1mas, and 7.9mas (+/- 0.3mas), respectively.
Simultaneous photometric observations yielded the bolometric fluxes. The
derived angular Rosseland radii and the bolometric fluxes allowed the
determination of effective temperatures. For instance, the effective
temperature of R Aql was determined to be 2970 +/- 110 K. A linear Rosseland
radius for R Aql of (250 +100/-60) Rsun was derived from the angular Rosseland
radius of 5.5mas +/- 0.2mas and the HIPPARCOS parallax of 4.73mas +/- 1.19mas.
The observations were compared with theoretical Mira star models of Bessel et
al. (1996) and Hofmann et al. (1998). The effective temperatures of the M-type
Miras and the linear radius of R Aql indicate fundamental mode pulsation.Comment: 12 pages, 4 postscript figure
Power Laws, Precursors and Predictability During Failure
We investigate the dynamics of a modified Burridge-Knopoff model by
introducing a dissipative term to mimic the bursts of acoustic emission (AE)
from rock samples. The model explains many features of the statistics of AE
signals observed in experiments such as the crossover in the exponent value
from relatively small amplitude AE signals to larger regime, and their
dependence on the pulling speed. Significantly, we find that the cumulative
energy dissipated identified with acoustic emission can be used to predict a
major slip event. We also find a data collapse of the acoustic activity for
several major slip events describable by a universal stretched exponential with
corrections in terms of time-to-failure.Comment: 7 pages, 6 figures, Final version with minor change
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
The Solar Neighborhood XV: Discovery of New High Proper Motion Stars with mu >= 0.4"/yr between Declinations -47 degrees and 00 degrees
We report the discovery of 152 new high proper motion systems (mu >= 0.4"/yr)
in the southern sky (Declination = -47 degrees to 00 degrees) brighter than
UKST plate R_{59F} =16.5 via our SuperCOSMOS-RECONS (SCR) search. This paper
complements Paper XII in The Solar Neighborhood series, which covered the
region from Declination = -90 degrees to -47 degrees and discussed all 147 new
systems from the southernmost phase of the search. Among the total of 299
systems from both papers, there are 148 (71 in Paper XII, 77 in this paper) new
systems moving faster than 0.5"/yr that are additions to the classic ``LHS''
(Luyten Half Second) sample. These constitute an 8% increase in the sample of
all stellar systems with mu >= 0.5"/yr in the southern sky.
As in Paper XII, distance estimates are provided for the systems reported
here based upon a combination of photographic plate magnitudes and 2MASS
photometry, assuming all stars are on the main sequence. Two SCR systems from
the portion of the sky included in this paper are anticipated to be within 10
pc, and an additional 23 are within 25 pc. In total, the results presented in
Paper XII and here for this SCR sweep of the entire southern sky include five
new systems within 10 pc and 38 more between 10 and 25 pc. The largest number
of nearby systems have been found in the slowest proper motion bin, 0.6"/yr >
mu >= 0.4"/yr, indicating that there may be a large population of low proper
motion systems very near the Sun.Comment: 36 pages, 5 figures, accepted for publication in Astronomical Journa
Frictional sliding without geometrical reflection symmetry
The dynamics of frictional interfaces play an important role in many physical
systems spanning a broad range of scales. It is well-known that frictional
interfaces separating two dissimilar materials couple interfacial slip and
normal stress variations, a coupling that has major implications on their
stability, failure mechanism and rupture directionality. In contrast,
interfaces separating identical materials are traditionally assumed not to
feature such a coupling due to symmetry considerations. We show, combining
theory and experiments, that interfaces which separate bodies made of
macroscopically identical materials, but lack geometrical reflection symmetry,
generically feature such a coupling. We discuss two applications of this novel
feature. First, we show that it accounts for a distinct, and previously
unexplained, experimentally observed weakening effect in frictional cracks.
Second, we demonstrate that it can destabilize frictional sliding which is
otherwise stable. The emerging framework is expected to find applications in a
broad range of systems.Comment: 14 pages, 5 figures + Supplementary Material. Minor change in the
title, extended analysis in the second par
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