921 research outputs found
Variable Stars: which Nyquist Frequency ?
In the analysis of variable stars, the problem of sampling is central. This
article focusses on the determination of the Nyquist frequency. It is well
defined in the case of regular sampling. However, the time series of variable
stars observations are generally unevenly sampled. Fourier analysis using the
spectral window furnishes some clues about the equivalent Nyquist frequency in
the irregular case. Often it is pushed very high, and thus very short periods
can be detected. A specific example is shown, drawn from MACHO databases.Comment: 4 pages, 5 figures, submitted to A&
Measurements of streaming motions of the Galactic bar with Red Clump Giants
We report a measurement of the streaming motion of the stars in the Galactic
bar with the Red Clump Giants (RCGs) using the data of the Optical
Gravitational Lensing Experiment II (OGLE-II). We measure the proper motion of
46,961 stars and divide RCGs into bright and faint sub-samples which on average
will be closer to the near and far side of the bar, respectively. We find that
the far-side RCGs (4,979 stars) have a proper motion of \Delta ~ 1.5 +-
0.11 mas yr^{-1} toward the negative l relative to the near-side RCGs (3,610
stars). This result can be explained by stars in the bar rotating around the
Galactic center in the same direction as the Sun with v_b ~ 100 km s^{-1}. In
the Disc Star (DS) and Red Giant (RG) samples, we do not find significant
difference between bright and faint sub-samples. For those samples \Delta
\~ 0.3 +- 0.14 mas yr^{-1} and ~ 0.03 +- 0.14 mas yr^{-1}, respectively. It is
likely that the average proper motion of RG stars is the same as that of the
Galactic center. The proper motion of DSs with respect to RGs is ~ 3.3 mas
yr^{-1} toward positive l. This value is consistent with the expectations for a
flat rotation curve and Solar motion with respect to local standard of rest.
RGs have proper motion approzimately equal to the average of bright and faint
RCGs, which implies that they are on average near the center of the bar. This
pilot project demonstrates that OGLE-II data may be used to study streaming
motions of stars in the Galactic bar. We intend to extend this work to all 49
OGLE-II fields in the Galactic bulge region.Comment: 7 pages, 9 figures, submitted to MNRA
Hunting for Runaways from the Orion Nebula Cluster
We use Gaia DR2 to hunt for runaway stars from the Orion Nebula Cluster
(ONC). We search a region extending 45{\deg} around the ONC and out to 1 kpc to
find sources that overlapped in angular position with the cluster in the last
~10 Myr. We find ~17,000 runaway/walkaway candidates satisfy this 2D traceback
condition. Most of these are expected to be contaminants, e.g., caused by
Galactic streaming motions of stars at different distances. We thus examine six
further tests to help identify real runaways, namely: (1) possessing young
stellar object (YSO) colors and magnitudes based on Gaia optical photometry;
(2) having IR excess consistent with YSOs based on 2MASS and WISE photometry;
(3) having a high degree of optical variability; (4) having closest approach
distances well constrained to within the cluster half-mass radius; (5) having
ejection directions that avoid the main Galactic streaming contamination zone;
and (6) having a required radial velocity (RV) for 3D overlap of reasonable
magnitude (or, for the 7% of candidates with measured RVs, satisfying 3D
traceback). Thirteen sources, not previously noted as Orion members, pass all
these tests, while another twelve are similarly promising, except they are in
the main Galactic streaming contamination zone. Among these 25 ejection
candidates, ten with measured RVs pass the most restrictive 3D traceback
condition. We present full lists of runaway/walkaway candidates, estimate the
high-velocity population ejected from the ONC and discuss its implications for
cluster formation theories via comparison with numerical simulations.Comment: 22 pages, 10 figures, and 5 tables. Accepted for publication in Ap
Notes on Cataglyphis Foerster, 1850 of the bicolor species-group in Israel, with description of a new species (Hymenoptera: Formicidae)
Five species of Cataglyphis belonging to the bicolor species-group are recorded from Israel, including a new species, C. israelensis n. sp. The relationships of the new species with other bicolor group species in the region, and relationships among species in the niger species-complex are discussed. The identification of isolated specimens belonging to the five Israeli species by morphological characters is investigated
Distances and parallax bias in Gaia DR2
We derive Bayesian distances for all stars in the radial velocity sample of Gaia DR2, and use the statistical method of SchĂśnrich, Binney & Asplund to validate the distances and test the Gaia parallaxes. In contrast to other methods, which rely on special sources, our method directly tests the distances to all stars in our sample. We find clear evidence for a near-linear trend of distance bias f with distance s, proving a parallax offset δp. On average, we find δp=â0.054mas (parallaxes in Gaia DR2 need to be increased) when accounting for the parallax uncertainty underestimate in the Gaia set (compared to δp=â0.048mas on the raw parallax errors), with negligible formal error and a systematic uncertainty of about 0.006masâ . The value is in concordance with results from asteroseismic measurements, but differs from the much lower bias found on quasar samples. We further use our method to compile a comprehensive set of quality cuts in colour, apparent magnitude, and astrometric parameters. Lastly, we find that for this sample δp appears to strongly depend on Ďp (when including the additional 0.043masâ ) with a statistical confidence far in excess of 10Ď and a proportionality factor close to 1, though the dependence varies somewhat with Ďp. Correcting for the Ďp dependence also resolves otherwise unexplained correlations of the offset with the number of observation periods nvis and ecliptic latitude. Every study using Gaia DR2 parallaxes/distances should investigate the sensitivity of its results on the parallax biases described here and â for fainter samples â in the DR2 astrometry paper
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