687 research outputs found
Resolving the pulsations of subdwarf B stars: HS 0039+4302, HS 0444+0458, and an examination of the group properties of resolved pulsators
We continue our program of single-site observations of pulsating subdwarf B
(sdB) stars and present the results of extensive time series photometry of HS
0039+4302 and HS 0444+0458. Both were observed at MDM Observatory during the
fall of 2005. We extend the number of known frequencies for HS 0039+4302 from 4
to 14 and discover one additional frequency for HS 0444+0458, bringing the
total to three. We perform standard tests to search for multiplet structure,
measure amplitude variations, and examine the frequency density to constrain
the mode degree .
Including the two stars in this paper, 23 pulsating sdB stars have received
follow-up observations designed to decipher their pulsation spectra. It is
worth an examination of what has been detected. We compare and contrast the
frequency content in terms of richness and range and the amplitudes with
regards to variability and diversity. We use this information to examine
observational correlations with the proposed pulsation mechanism as
well as alternative theories.Comment: 32 pages, 18 figures, 7 tables. Accepted for publication in MNRA
The Age of the Oldest Stars in the Local Galactic Disk From Hipparcos Parallaxes of G and K Subgiants
We review the history of the discovery of field subgiant stars and their
importance in the age dating of the Galactic disk. We use the cataloged data
from the Hipparcos satellite in this latter capacity. Based on Hipparcos
parallaxes accurate to 10% or better, the absolute magnitude of the lower
envelope of the nearly horizontal subgiant sequence for field stars in the H-R
diagram for B-V colors between 0.85 and 1.05 is measured to be M_V = 4.03 +/-
0.06. The age of the field stars in the solar neighborhood is found to be 7.9
+/- 0.7 Gyr by fitting the theoretical isochrones for [Fe/H] = +0.37 to the
lower envelope of the Hipparcos subgiants. The same grid of isochrones yields
ages, in turn, of 4.0 +/- 0.2 Gyr, 6.2 +/- 0.5 Gyr, and 7.5 to 10 Gyr for the
old Galactic clusters M67, NGC188, and NGC6791. The ages of both the Galactic
disk in the solar neighborhood and of NGC6791 are, nevertheless, likely between
3 and 5 Gyr younger than the oldest halo globular clusters, which have ages of
13.5 Gyr. The most significant results are (1) the supermetallicity of the
oldest local disk stars, and (2) the large age difference between the most
metal-poor component of the halo and the thick and thin disk in the solar
neighborhood. These facts are undoubtedly related and pose again the problem of
the proper scenario for the timing of events in the formation of the halo and
the Galactic disk in the solar neighborhood. [Abstract Abridged]Comment: 44 pages, 12 Figures; accepted for publication in PASP; high
resolution versions of Figures 1, 2, 6 and 9 available at
http://bubba.ucdavis.edu/~lubin/Sandage
The nearest young moving groups
The latest results in the research of forming planetary systems have led
several authors to compile a sample of candidates for searching for planets in
the vicinity of the sun. Young stellar associations are indeed excellent
laboratories for this study, but some of them are not close enough to allow the
detection of planets through adaptive optics techniques. However, the existence
of very close young moving groups can solve this problem. Here we have compiled
the members of the nearest young moving groups, as well as a list of new
candidates from our catalogue of late-type stars possible members of young
stellar kinematic groups, studying their membership through spectroscopic and
photometric criteria.Comment: Latex file with 16 pages, 4 figures. Available at
http://www.ucm.es/info/Astrof/invest/actividad/skg/skg_sag.html Accepted for
publication in: The Astrophysical Journal (ApJ
`Oumuamua as a messenger from the Local Association
7 pages, one table, two figures, accepted for publication by ApJL. © 2018. The American Astronomical Society. All rights reserved.With a hyperbolic trajectory around the Sun, 'Oumuamua is the first confirmed interstellar object. However, its origin is poorly known. By simulating the orbits of 0.23 million local stars, we find 109 encounters with periastron less than 5 pc. 'Oumuamua's low peculiar velocity is suggestive of its origin from a young stellar association with similar velocity. In particular, we find that 'Oumuamua would have had slow encounters with at least five young stars belonging to the Local Association, thus suggesting these as plausible sites for formation and ejection. In addition to an extremely elongated shape, the available observational data for 'Oumuamua indicates a red color, suggestive of a potentially organic-rich and activity-free surface. These characteristics seem consistent with formation through energetic collisions between planets and debris objects in the middle part of a young stellar system. We estimate an abundance of at least 6.0 à 10 -3 au -3 for such interstellar objects with mean diameter larger than 100 m and find that it is likely that most of them will be ejected into the Galactic halo. Our Bayesian analysis of the available light curves indicates a rotation period of 6.96 +1.45 -0.39, which is consistent with the estimation by Meech et al. and shorter than those in other literature. The codes and results are available on GitHub (https://github.com/phillippro/Oumuamua).Peer reviewe
Blue Straggler Stars: Early Observations that Failed to Solve the Problem
In this chapter, I describe early ideas on blue stragglers, and various
observations (some published, some not) that promised but failed to resolve the
question of their origin. I review the data and ideas that were circulating
from Allan Sandage's original discovery in 1953 of "anomalous blue stars" in
the globular cluster M3, up until about 1992, when what seems to have been the
only previous meeting devoted to Blue Straggler Stars (BSSs) was held at the
Space Telescope Science Institute.Comment: Chapter 2, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
Low Luminosity Companions to White Dwarfs
This paper presents results of a near-infrared imaging survey for low mass
stellar and substellar companions to white dwarfs. A wide field proper motion
survey of 261 white dwarfs was capable of directly detecting companions at
orbital separations between and 5000 AU with masses as low as 0.05
, while a deep near field search of 86 white dwarfs was capable of
directly detecting companions at separations between and 1100 AU with
masses as low as 0.02 . Additionally, all white dwarf targets were
examined for near-infrared excess emission, a technique capable of detecting
companions at arbitrarily close separations down to masses of 0.05 .
No brown dwarf candidates were detected, which implies a brown dwarf
companion fraction of % for white dwarfs. In contrast, the stellar
companion fraction of white dwarfs as measured by this survey is 22%,
uncorrected for bias. Moreover, most of the known and suspected stellar
companions to white dwarfs are low mass stars whose masses are only slightly
greater than the masses of brown dwarfs. Twenty previously unknown stellar
companions were detected, five of which are confirmed or likely white dwarfs
themselves, while fifteen are confirmed or likely low mass stars.
Similar to the distribution of cool field dwarfs as a function of spectral
type, the number of cool unevolved dwarf companions peaks at mid-M type. Based
on the present work, relative to this peak, field L dwarfs appear to be roughly
2-3 times more abundant than companion L dwarfs. Additionally, there is no
evidence that the initial companion masses have been altered by post main
sequence binary interactions.Comment: 149 pages, 59 figures, 11 tables, accepted to ApJ Supplement
Galactic bulge formation as a maximum intensity starburst
Properties of normal galactic star formation, including the density
dependence, threshold density, turbulent scaling relations, and clustering
properties, are applied to the formation of galactic bulges. One important
difference is that the bulge potential well is too deep to have allowed
self-regulation or blow-out by the pressures from young stars, unlike galactic
disks or dwarf galaxies. As a result, bulge formation should have been at the
maximum rate, which is such that most of the gas would get converted into stars
in only a few dynamical time scales, or ~10^8 years. The gas accretion phase
can be longer than this, but once the critical density is reached, which
depends primarily on the total virial density from dark matter, the formation
of stars in the bulge should have been extremely rapid. Such three-dimensional
star formation should also have formed many clusters, like normal disk star
formation today. Some of these clusters may have survived as old globulars, but
most got dispersed, although they might still be observable as concentrated
streams in phase space.Comment: 10 pages, 1 figure, scheduled for ApJ, vol. 517, May 20, 199
Stars of extragalactic origin in the solar neighborhood
We computed the spatial velocities and the galactic orbital elements using
Hipparcos data for 77 nearest main-sequence F-G-stars with published the iron,
magnesium, and europium abundances determined from high dispersion spectra and
with the ages estimated from theoretical isochrones. A comparison with the
orbital elements of the globular clusters that are known was accreted by our
Galaxy in the past reveals stars of extragalactic origin. We show that the
relative elemental abundance ratios of r- and \alpha- elements in all the
accreted stars differ sharply from those in the stars that are genetically
associated with the Galaxy. According to current theoretical models, europium
is produced mainly in low mass Type II supernovae (SNe II), while magnesium is
synthesized in larger amounts in high mass SN II progenitors. Since all the old
accreted stars of our sample exhibit a significant Eu overabundance relative to
Mg, we conclude that the maximum masses of the SNII progenitors outside the
Galaxy were much lower than those inside it are. On the other hand, only a
small number of young accreted stars exhibit low negative ratios .
The delay of primordial star formation burst and the explosions of high mass
SNe II in a relatively small part of extragalactic space can explain this
situation. We provide evidence that the interstellar medium was weakly mixed at
the early evolutionary stages of the Galaxy formed from a single proto-galactic
cloud and that the maximum mass of the SN II progenitors increased in it with
time simultaneously with the increase in mean metallicity.Comment: Accepted for 2004, Astronomy Letters, Vol. 30, No. 3, P.148-158 15
pages, 3 figure
A qualitative exploration of memory cuing by personal items in the home
© The Author(s) 2017. We are surrounded by personal items that can trigger memories, such as photos, souvenirs and heirlooms. Also during holidays, we collect items to remind us of the events, but not all bring back memories to the same extent. Therefore, we explored peoplesâ responses to personal items related to a holiday, using the home tour interviewing method. In total, 63 accounts of cuing responses from nine home tours were analysed using thematic analysis. This resulted in four types of cuing responses: (a) âno-memoryâ responses, (b) âknowâ responses, (c) âmemory evoked think or feelâ responses and (d) ârememberâ responses. For each of these cuing response categories, we looked into the types of items and their characteristics. Furthermore, we found that some items can evoke multiple memories. The majority of the memoriesâ content refers to events close to the moment of acquiring the item
Atopic conditions and brain tumor risk in children and adolescentsâan international case-control study (CEFALO)
In this study, atopic conditions were not associated with risk of brain tumors in children and adolescents or of glioma in particular. Results are not consistent with findings for adult glioma, possibly explained by a different distribution of histological subtypes. Only a few studies on atopic conditions and pediatric brain tumors are currently available, and the evidence is conflictin
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