6,749 research outputs found
Search for nearby stars among proper motion stars selected by optical-to-infrared photometry. II. Two late M dwarfs within 10 pc
We have identified two late M dwarfs within 10 parsecs of the Sun, by
cross-correlating the Luyten NLTT catalogue of stars with proper motions larger
than 0.18 arcsec/yr, with objects lacking optical identification in the 2MASS
data base. The 2MASS photometry was then combined with improved optical
photometry obtained from the SuperCOSMOS Sky Surveys. The two objects (LP775-31
and LP655-48) have extremely red optical-to-infrared colours ((R-K)~7) and very
bright infrared magnitudes (K_s<10): follow-up optical spectroscopy with the
ESO 3.6-m telescope gave spectral types of M8.0 and M7.5 dwarfs, respectively.
Comparison of their near-infrared magnitudes with the absolute magnitudes of
known M8 and M7.5 dwarfs with measured trigonometric parallaxes yields
spectroscopic distance estimates of 6.4+/-1.4 parsecs and 8.0+/-1.6 parsecs for
LP775-31 and LP655-48, respectively. In contrast, Cruz & Reid (2002) recently
determined spectral types of M6 for both objects, and commensurately larger
distances of 11.3+/-1.3 parsecs and 15.3+/-2.6 parsecs. LP655-48 is also a
bright X-ray source (1RXS J044022.8-053020). With only a few late M dwarfs
previously known within 10 parsecs, these two objects represent an important
addition to the census of the Solar neighbourhood.Comment: Astronomy & Astrophysics (Letters), in press; 5 pages, 1 figure, uses
aa.cls version 5.
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.Comment: 4 pages, 4 figures (Fig.1a-d available as jpg files), accepted for
publication in Astronomy & Astrophysics Letter
A U-band survey of brown dwarfs in the Taurus Molecular Cloud with the XMM-Newton Optical/UV Monitor
We aim to characterize the U-band variability of young brown dwarfs in the
Taurus Molecular Cloud and discuss its origin. We used the XMM-Newton Extended
Survey of the Taurus Molecular Cloud, where a sample of 11 young bona fide
brown dwarfs (spectral type later than M6) were observed simultaneously in
X-rays with XMM-Newton and in the U-band with the XMM-Newton Optical/UV Monitor
(OM). We obtained upper limits to the U-band emission of 10 brown dwarfs
(U>19.6-20.6 mag), whereas 2MASSJ04141188+2811535 was detected in the U-band.
Remarkably, the magnitude of this brown dwarf increased regularly from U~19.5
mag at the beginning of the observation, peaked 6h later at U~18.4 mag, and
then decreased to U~18.65 mag in the next 2h. The first OM U-band measurement
is consistent with the quiescent level observed about one year later thanks to
ground follow-up observations. This brown dwarf was not detected in X-rays by
XMM-Newton during the OM observation. We discuss the possible sources of U-band
variability for this young brown dwarf, namely a magnetic flare, non-steady
accretion onto the substellar surface, and rotational modulation of a hot spot.
We conclude that this event is related to accretion from a circumsubstellar
disk, where the mass accretion rate was about a factor of 3 higher than during
the quiescent level.Comment: 6 pages and 4 Figures. Accepted by A&A, to appear in a special
section/issue dedicated to the XMM-Newton Extended Survey of the Taurus
Molecular Cloud (XEST
Electrochemistry reveals archaeological materials
The characterization of materials constituting cultural artefacts is a challenging step in their conservation, due to the objectâs uniqueness and the reduced number of conservation institutes able to supply non-destructive analysis. We propose an alternative analytical tool, which combines accessibility (low cost and portable) and high sensitivity, based on electrochemical linear sweep voltammetry (LSV) with paraffin impregnated graphite electrode (PIGE). To investigate the composition of âwhite alloysâ that certainly have been used as decoration on copper-based Roman fibulae, sampling was done very locally by gently rubbing the selected areas with the PIGE. LSV results evidence the presence of silver, lead, and tin, supporting the argument provided by typological analysis that these metals were used for decoration
The execution of systematic measurements on plane cascades
The present state of development of the experimental technique regarding the flow through cascades and several points to be specially observed in the design of cascade wind tunnels were discussed. The equations required for the evaluation of the momentum measurements in two dimensional flow through cascades were developed. Regarding the effect of the jet contraction due to the boundary layer along the side walls a simple method for correction was also given in order to obtain two dimensional flow characteristics. Also given were the equations for the evaluation of the pressure distribution measurements. Another contribution was made regarding the presentation of the test results in the form of nondimensional quantities. The results of systematic measurements of cascades with symmetrical aerofoil were reported, and the above suggested method was applied for the evaluation of the measurements
Epsilon Indi B: a new benchmark T dwarf
We have identified a new early T dwarf only 3.6pc from the Sun, as a common
proper motion companion (separation 1459AU) to the K5V star Epsilon Indi
(HD209100). As such, Epsilon Indi B is one of the highest proper motion sources
outside the solar system (~4.7 arcsec/yr), part of one of the twenty nearest
stellar systems, and the nearest brown dwarf to the Sun. Optical photometry
obtained from the SuperCOSMOS Sky Survey was combined with approximate infrared
photometry from the 2MASS Quicklook survey data release, yielding colours for
the source typical of early T dwarfs. Follow up infrared spectroscopy using the
ESO NTT and SOFI confirmed its spectral type to be T2.5+/-0.5. With Ks=11.2,
Epsilon Indi B is 1.7 magnitudes brighter than any previously known T dwarf and
4 magnitudes brighter than the typical object in its class, making it highly
amenable to detailed study. Also, as a companion to a bright nearby star, it
has a precisely known distance (3.626pc) and relatively well-known age
(0.8-2Gyr), allowing us to estimate its luminosity as logL/Lsun=-4.67, its
effective temperature as 1260K, and its mass as ~40-60Mjup. Epsilon Indi B
represents an important addition to the census of the Solar neighbourhood and,
equally importantly, a new benchmark object in our understanding of substellar
objects.Comment: Accepted by A&A (Letters); 5 pages, 3 figure
Global survey of star clusters in the Milky Way II. The catalogue of basic parameters
Although they are the main constituents of the Galactic disk population, for
half of the open clusters in the Milky Way reported in the literature nothing
is known except the raw position and an approximate size. The main goal of this
study is to determine a full set of uniform spatial, structural, kinematic, and
astrophysical parameters for as many known open clusters as possible. On the
basis of stellar data from PPMXL and 2MASS, we used a dedicated data-processing
pipeline to determine kinematic and photometric membership probabilities for
stars in a cluster region. For an input list of 3784 targets from the
literature, we confirm that 3006 are real objects, the vast majority of them
are open clusters, but associations and globular clusters are also present. For
each confirmed object we determined the exact position of the cluster centre,
the apparent size, proper motion, distance, colour excess, and age. For about
1500 clusters, these basic astrophysical parameters have been determined for
the first time. For the bulk of the clusters we also derived the tidal radius.
We estimated additionally average radial velocities for more than 30% of the
confirmed clusters. The present sample (called MWSC) reaches both the central
parts of the Milky Way and its outer regions. It is almost complete up to 1.8
kpc from the Sun and also covers neighbouring spiral arms. However, for a small
subset of the oldest open clusters () we found some evidence
of incompleteness within about 1 kpc from the Sun.Comment: 8 pages, 5 figures, accepted for publication in Astronomy and
Astrophysic
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