11,117 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.
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
Surface deformation and elasticity studies in the Virgin Islands
The report consists of four sections. The first section describes tilt and leveling measurements on Anegada, the most northerly of the British Virgin Islands; the second section contains a discussion of sea-level measurements that were initiated in the region and which played a significant role in the development of a network of sea-level monitors now telemetered via satellite from the Alaskan Shumagin Islands. The third part of the report is a brief description of surface deformation measurements in Iceland using equipment and techniques developed by the subject grant. The final part of the report describes the predicted effects of block surface fragmentation in tectonic areas on the measurement of tilt and strain
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
Effects of moderate abundance changes on the atmospheric structure and colours of Mira variables (Research Note)
Aims. We study the effects of moderate deviations from solar abundances upon
the atmospheric structure and colours of typical Mira variables. Methods. We
present two model series of dynamical opacity-sampling models of Mira variables
which have (1) 1 solar metallicity 3 and (2) "mild" S-type C/O abundance ratio
([C/O]=0.9) with typical Zr enhancement (solar +1.0). These series are compared
to a previously studied solar-abundance series which has similar fundamental
parameters (mass, luminosity, period, radius) that are close to those of o Cet.
Results. Both series show noticeable effects of abundance upon stratifications
and infrared colours but cycle-to-cycle differences mask these effects at most
pulsation phases, with the exception of a narrow-water-filter colour near
minimum phase.Comment: 4 pages, 3 figures, accepted for A&
The Clinton welfare reform plan: Will it end poverty as we know it
The central elements in President Clinton's proposal to reform the welfare system are: increasing the earned income tax credit, improving the child support system, educating and training the poor, and limiting the amount of time people can receive assistance. The authors commend the first two components of the president's plan but question the likely effectiveness of the last two: even with the education, training, and child care programs that the president has proposed, few welfare recipients will be able to command wages that would lift them out of poverty, and successful education and training programs would cost more than the government appears willing to spend. They recommend that the president consider giving tax credits to, and subsidizing the wages paid by, employers who hire low-wage workers and assist young people and poor families to save for future opportunities. In their view, poverty will not be alleviated by only getting tough on welfare recipients; instead, labor market interventions should be adopted so as to expand opportunities for low-wage, low-skilled workers.
The Rotation of Young Low-Mass Stars and Brown Dwarfs
We review the current state of our knowledge concerning the rotation and
angular momentum evolution of young stellar objects and brown dwarfs from a
primarily observational view point. Periods are typically accurate to 1% and
available for about 1700 stars and 30 brown dwarfs in young clusters.
Discussion of angular momentum evolution also requires knowledge of stellar
radii, which are poorly known for pre-main sequence stars. It is clear that
rotation rates at a given age depend strongly on mass; higher mass stars
(0.4-1.2 M) have longer periods than lower mass stars and brown dwarfs.
On the other hand, specific angular momentum is approximately independent of
mass for low mass pre-main sequence stars and young brown dwarfs. A spread of
about a factor of 30 is seen at any given mass and age. The evolution of
rotation of solar-like stars during the first 100 Myr is discussed. A broad,
bimodal distribution exists at the earliest observable phases (1 Myr) for
stars more massive than 0.4 M. The rapid rotators (50-60% of the
sample) evolve to the ZAMS with little or no angular momentum loss. The slow
rotators continue to lose substantial amounts of angular momentum for up to 5
Myr, creating the even broader bimodal distribution characteristic of 30-120
Myr old clusters. Accretion disk signatures are more prevalent among slowly
rotating PMS stars, indicating a connection between accretion and rotation.
Disks appear to influence rotation for, at most, 5 Myr, and considerably
less than that for the majority of stars. If the dense clusters studied so far
are an accurate guide, then the typical solar-like star may have only 1
Myr for this task. It appears that both disk interactions and stellar winds are
less efficient at braking these objects.Comment: Review chapter for Protostars and Planets V. 15 page and 8 figure
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
A novel paradigm for testing the initial coding of hierarchical relationships within the medial temporal lobe in a circuit specific manner
The Medial Temporal Lobe (MTL) is central to spatial navigation and to the processing of conceptual associations, functions which can be implemented via the grid cell system. Evidence exists that also hierarchical processing - an integral part of cognitive capacities such as language, motor action and sequential planning - draws on the MTL. This raises the question of whether hierarchical processing shares the same cellular substrate with spatial navigation and concept association formation. Here we present two novel tasks (hierarchy and control) specifically designed to test (using fMRI) whether grid cells also support the representation of hierarchical relations. We present the first results of their behavioral validation with respect to specificity, and show that our hierarchical task, but not the control task, correlates well with other tasks that necessitate hierarchical processing (Tower of Hanoi and a Visual Recursion Task). Furthermore, we show that some of these effects remain even when removing the shared variance that can be explained by a range of unspecific factors. This gives reason to believe that our task is a valid method for probing the relationship between grid cells and hierarchical processing
Wavelength dependence of angular diameters of M giants: an observational perspective
We discuss the wavelength dependence of angular diameters of M giants from an
observational perspective. Observers cannot directly measure an optical-depth
radius for a star, despite this being a common theoretical definition. Instead,
they can use an interferometer to measure the square of the fringe visibility.
We present new plots of the wavelength-dependent centre-to-limb variation (CLV)
of intensity of the stellar disk as well as visibility for Mira and non-Mira M
giant models. We use the terms ``CLV spectra'' and ``visibility spectra'' for
these plots. We discuss a model-predicted extreme limb-darkening effect (also
called the narrow-bright-core effect) in very strong TiO bands which can lead
to a misinterpretation of the size of a star in these bands. We find no
evidence as yet that this effect occurs in real stars. Our CLV spectra can
explain the similarity in visibilities of R Dor (M8IIIe) that have been
observed recently despite the use of two different passbands. We compare
several observations with models and find the models generally under-estimate
the observed variation in visibility with wavelength. We present CLV and
visibility spectra for a model that is applicable to the M supergiant alpha
Ori.Comment: 16 pages with figures. Accepted by MNRA
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