713 research outputs found
The Solar Neighborhood VIII: Discovery of New High Proper Motion Nearby Stars Using the SuperCOSMOS Sky Survey
Five new objects with proper motions between 1.0 arcsec/yr and 2.6 arcsec/yr
have been discovered via a new RECONS search for high proper motion stars
utilizing the SuperCOSMOS Sky Survey. The first portion of the search,
discussed here, is centered on the south celestial pole and covers declinations
-90 degrees to -57.5 degrees.
Photographic photometry from SuperCOSMOS and JHKs near-infrared photometry
from 2MASS for stars nearer than 10 pc are combined to provide a suite of new
M_Ks-color relations useful for estimating distances to main sequence stars.
These relations are then used to derive distances to the new proper motion
objects as well as previously known stars with mu >= 1.0 arcsec/yr (many of
which have no trigonometric parallaxes) recovered during this phase of the
survey.
Four of the five new stars have red dwarf colors, while one is a nearby white
dwarf. Two of the red dwarfs are likely to be within the RECONS 10 pc sample,
and the white dwarf probably lies between 15 and 25 pc. Among the 23 known
stars recovered during the search, there are three additional candidates for
the RECONS sample that have no trigonometric parallaxes.Comment: 17 pages, 5 figures; accepted for publication in Astronomy Journa
The Solar Neighborhood. XXXIV. A Search for Planets Orbiting Nearby M Dwarfs using Astrometry
Astrometric measurements are presented for seven nearby stars with previously
detected planets: six M dwarfs (GJ 317, GJ 667C, GJ 581, GJ 849, GJ 876, and GJ
1214) and one K dwarf (BD 10 3166). Measurements are also presented for six
additional nearby M dwarfs without known planets, but which are more favorable
to astrometric detections of low mass companions, as well as three binary
systems for which we provide astrometric orbit solutions. Observations have
baselines of three to thirteen years, and were made as part of the RECONS
long-term astrometry and photometry program at the CTIO/SMARTS 0.9m telescope.
We provide trigonometric parallaxes and proper motions for all 16 systems, and
perform an extensive analysis of the astrometric residuals to determine the
minimum detectable companion mass for the 12 M dwarfs not having close stellar
secondaries. For the six M dwarfs with known planets, we are not sensitive to
planets, but can rule out the presence of all but the least massive brown
dwarfs at periods of 2 - 12 years. For the six more astrometrically favorable M
dwarfs, we conclude that none have brown dwarf companions, and are sensitive to
companions with masses as low as 1 for periods longer than two years.
In particular, we conclude that Proxima Centauri has no Jovian companions at
orbital periods of 2 - 12 years. These results complement previously published
M dwarf planet occurrence rates by providing astrometrically determined upper
mass limits on potential super-Jupiter companions at orbits of two years and
longer. As part of a continuing survey, these results are consistent with the
paucity of super-Jupiter and brown dwarf companions we find among the over 250
red dwarfs within 25 pc observed longer than five years in our astrometric
program.Comment: 18 pages, 5 figures, 4 tables, accepted for publication in A
New Hipparcos-based Parallaxes for 424 Dim Stars
We present a catalog of 424 common proper motion companions to Hipparcos
stars with good (>3 sigma) parallaxes, thereby effectively providing new
parallaxes for these companions. Compared to stars in the Hipparcos catalog,
these stars are substantially dimmer. The catalog includes 20 WDs and an
additional 29 stars with M_V>14, the great majority of the latter being M
dwarfs.Comment: Submitted to ApJS, 20 page
The Solar Neighborhood XVII: Parallax Results from the CTIOPI 0.9m Program -- Twenty New Members of the RECONS 10 Parsec Sample
Astrometric measurements for 25 red dwarf systems are presented, including
the first definitive trigonometric parallaxes for 20 systems within 10 pc of
the Sun, the horizon of the RECONS sample. The three nearest systems that had
no previous trigonometric parallaxes (other than perhaps rough preliminary
efforts) are SO 0253+1652 (3.84 +/- 0.04 pc, the 23rd nearest system), SCR
1845-6357 AB (3.85 +/- 0.02 pc, 24th), and LHS 1723 (5.32 +/- 0.04 pc, 56th).
In total, seven of the systems reported here rank among the nearest 100 stellar
systems. Supporting photometric and spectroscopic observations have been made
to provide full characterization of the systems, including complete VRIJHK
photometry and spectral types. A study of the variability of 27 targets reveals
six obvious variable stars, including GJ 1207, for which we observed a flare
event in the V band that caused it to brighten by 1.7 mag.
Improved parallaxes for GJ 54 AB and GJ 1061, both important members of the
10 pc sample, are also reported. Definitive parallaxes for GJ 1001 A, GJ 633,
and GJ 2130 ABC, all of which have been reported to be within 10 pc, indicate
that they are beyond 10 pc. From the analysis of systems with (previously) high
trigonometric parallax errors, we conclude that parallaxes with errors in
excess of 10 mas are insufficiently reliable for inclusion in the RECONS
sample. The cumulative total of new additions to the 10 pc sample since 2000 is
now 34 systems -- 28 by the RECONS team and six by other groups. This total
represents a net increase of 16% in the number of stellar systems reliably
known to be nearer than 10 pc.Comment: 33 pages, including 3 figures and 3 table
Mind the Gap I: H Activity of M Dwarfs Near the Partially/Fully Convective Boundary and a New H Emission Deficiency Zone on the Main Sequence
Since identifying the gap in the H-R Diagram (HRD) marking the transition
between partially and fully convective interiors, a unique type of slowly
pulsating M dwarf has been proposed. These unstable M dwarfs provide new
laboratories in which to understand how changing interior structures result in
potentially observable activity at the surface. In this work, we report the
results of the largest high-resolution spectroscopic H emission survey
to date spanning this transition region, including 480 M dwarfs observed using
the CHIRON spectrograph at CTIO/SMARTS 1.5-m. We find that M dwarfs with
H in emission are almost entirely found 0 to 0.5 magnitude above the
top edge of the gap in the HRD, whereas effectively no stars in and below the
gap show emission. Thus, the top edge of the gap marks a relatively sharp
activity transition and there is no anomalous H activity for stars in
the gap. We also identify a new region at 10.3 10.8 on the main
sequence where fewer M dwarfs exhibit H emission compared to M dwarfs
above and below this magnitude range. Careful evaluation of literature results
indicates that 1) rotation and H activity distributions on the main
sequence are closely related, and 2) fewer stars in this absolute magnitude
range rotate in less than 13 days than populations surrounding this
region. This result suggests that the most massive fully convective stars lose
their angular momentum faster than both partially convective stars and less
massive fully convective stars.Comment: 30 pages, 18 figures, and 6 table. Submitted to A
Rational isogenies from irrational endomorphisms
In this paper, we introduce a polynomial-time algorithm to compute a connecting -ideal between two supersingular elliptic curves over with common -endomorphism ring , given a description of their full endomorphism rings. This algorithm provides a reduction of the security of the CSIDH cryptosystem to the problem of computing endomorphism rings of supersingular elliptic curves. A similar reduction for SIDH appeared at Asiacrypt 2016, but relies on totally different techniques. Furthermore, we also show that any supersingular elliptic curve constructed using the complex-multiplication method can be located precisely in the supersingular isogeny graph by explicitly deriving a path to a known base curve. This result prohibits the use of such curves as a building block for a hash function into the supersingular isogeny graph
Sarawak Peat Characteristics and Heat Treatment
Peat layer is extensively scattered over the land of Malaysia. In Sarawak alone, peatland represents 13 percent (about 1.66
million hectares) of Sarawakâs total land area. They are present, mostly in low-lying areas; with in some areas, peat exceeding 10 m in depth. During past few decades, the demands on development of land were expanded into the swamp and deep peat areas which cannot be avoided. Thus proper management and construction practices should be emphasized, in order to overcome consequential occurrence of ground subsidence problems. The objectives of this study are to determine the characteristics of Sarawak peat, their empirical correlations
as well as the effect of heat treatment on peat. The samples were taken from Matang, Batu Kawa, and Kota Samarahan sites, in Sarawak. The characteristic tests consist of degree of humidification, loss on ignition, Atterberg limit, particle density, moisture content and pH value. The results recorded high moisture content and organic content of Sarawak peat. It is also being categorized under the hemic group with pH values ranging from 3 to 4. The heat treatments with temperatures ranging from 100°C to 400°C were used on the peat samples.
Samples collected were undergoing heat treatment and changes to its physical characteristic were compared with the original Sarawak peat. It was found that the heat treatments do influence the physical properties of Sarawak peat and have shown significant reduction in the compression index determined through the empirical correlations
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