935 research outputs found
The Open Cluster Distance Scale: A New Empirical Approach
We present new BVRI photometry for a sample of 54 local G and K stars with accurate Hipparcos parallaxes in the metallicity range -0.4 < [Fe/H] < +0.3. We use this sample to develop a completely model-independent main sequence (MS) fitting method which we apply to 4 open clusters - the Hyades, Praesepe, the Pleiades and NGC 2516 - which all have direct Hipparcos parallax distance determinations. Comparison of our MS-fitting results with distances derived from Hipparcos parallaxes enables us to explore whether the discrepancy between the Hipparcos distance scale and other MS-fitting methods found for some clusters is a consequence of model assumptions. We find good agreement between our results and the Hipparcos ones for the Hyades and Praesepe. For the Pleiades and NGC 2516, when adopting the solar abundance determined from spectroscopy, we find significant disagreement at a level similar to that found by other MS-fitting studies. However, the colour-colour relationship for both these clusters suggests that their metallicity is significantly subsolar. Since the MS-fitting method relies on matching the cluster colours to a template MS, we argue that, when applying this method, the appropriate metallicity to adopt is the photometric subsolar one, not the solar abundance indicated by spectroscopy. Adopting photometric metallicities for all 4 clusters, we find complete agreement with the Hipparcos results and hence we conclude that the mismatch between the spectroscopic and photometric abundances for the Pleiades and NGC 2516 is responsible for the discrepancies in distance estimates found by previous studies. The origin of this mismatch in abundance scales remains an unsolved problem and some possible causes are discussed
Orbital Characteristics of the Subdwarf-B and F V Star Binary EC~20117-4014(=V4640 Sgr)
Among the competing evolution theories for subdwarf-B (sdB) stars is the
binary evolution scenario. EC~20117-4014 (=V4640~Sgr) is a spectroscopic binary
system consisting of a pulsating sdB star and a late F main-sequence companion
(O'Donoghue et al. 1997), however the period and the orbit semi-major axes have
not been precisely determined. This paper presents orbital characteristics of
the EC 20117-4014 binary system using 20 years of photometric data. Periodic
Observed minus Calculated (O-C) variations were detected in the two highest
amplitude pulsations identified in the EC 20117-4014 power spectrum, indicating
the binary system's precise orbital period (P = 792.3 days) and the
light-travel time amplitude (A = 468.9 s). This binary shows no significant
orbital eccentricity and the upper limit of the eccentricity is 0.025 (using 3
as an upper limit). This upper limit of the eccentricity is the lowest
among all wide sdB binaries with known orbital parameters. This analysis
indicated that the sdB is likely to have lost its hydrogen envelope through
stable Roche lobe overflow, thus supporting hypotheses for the origin of sdB
stars. In addition to those results, the underlying pulsation period change
obtained from the photometric data was = 5.4 (0.7)
d d, which shows that the sdB is just before the end of the
core helium-burning phase
Accelerated Stress Testing of Hydrocarbon-Based Encapsulants for Medium-Concentration CPV Applications
Concentrating photovoltaic (CPV) systems have great potential to reduce photovoltaic (PV) electricity costs because of the relatively low cost of optical components as compared to PV cells. A transparent polymeric material is used to optically couple the PV cell to optical components and is thus exposed to the concentrated light source at elevated temperatures. In this work polymeric encapsulant materials are positioned close to a Xenon arc lamp to expose them to ultraviolet radiation (UV) that is about 42 times as intense as sunlight. Furthermore, different glass types are used as filters to modify the spectral distribution of light in the UV range. A strong sensitivity of non-silicone-based encapsulants to light below ~350 nm is demonstrated. Of all the materials examined in this study, the polydimethyl silicone samples performed the best. The next best material was an ionomer which maintained optical transmission but became photo-oxidized where exposed to the atmosphere
Galactic Structure Toward the Carina Tangent
This investigation presents a photometric study of the Galactic structure
toward the Carina arm tangent. The field is located between 280 deg and 286 deg
galactic longitude and -4 deg to 4 deg galactic latitude. All currently
available uvbybeta data is used to obtain homogeneous color excesses and
distances for more than 260 stars of spectral types O to G. We present revised
distances and average extinction for the open clusters and cluster candidates
NGC 3293, NGC 3114, Loden 46 and Loden 112. The cluster candidate Loden 112
appears to be a very compact group at a true distance modulus of 11.06 +\- 0.11
(s.e.) (1629 +84,-80 pc), significantly closer than previous estimates. We
found other OB stars at that same distance and, based on their proper motions,
suggest a new OB association at coordinates 282 deg < l < 285 deg, -2 deg < b <
2 deg. Utilizing BV photometry and spectral classification of the known O-type
stars in the very young open cluster Wd 2 we provide a new distance estimate of
14.13 +\-0.16 (s.e.) (6698 +512,-475 pc), in excellent agreement with recent
distance determinations to the giant molecular structures in this direction. We
also discuss a possible connection between the HII region RCW 45 and the
highly-reddened B+ star CPD -55 3036 and provide a revised distance for the
luminous blue variable HR Car.Comment: accepted to PAS
The Solar Neighborhood. XIX. Discovery and Characterization of 33 New Nearby White Dwarf Systems
We present spectra for 33 previously unclassified white dwarf systems
brighter than V = 17 primarily in the southern hemisphere. Of these new
systems, 26 are DA, 4 are DC, 2 are DZ, and 1 is DQ. We suspect three of these
systems are unresolved double degenerates. We obtained VRI photometry for these
33 objects as well as for 23 known white dwarf systems without trigonometric
parallaxes, also primarily in the southern hemisphere. For the 56 objects, we
converted the photometry values to fluxes and fit them to a spectral energy
distribution using the spectroscopy to determine which model to use (i.e. pure
hydrogen, pure helium, or metal-rich helium), resulting in estimates of
effective temperature and distance. Eight of the new and 12 known systems are
estimated to be within the NStars and Catalogue of Nearby Stars (CNS) horizons
of 25 pc, constituting a potential 18% increase in the nearby white dwarf
sample. Trigonometric parallax determinations are underway via CTIOPI for these
20 systems.
One of the DCs is cool so that it displays absorption in the near infrared.
Using the distance determined via trigonometric parallax, we are able to
constrain the model-dependent physical parameters and find that this object is
most likely a mixed H/He atmosphere white dwarf similar to other cool white
dwarfs identified in recent years with significant absorption in the infrared
due to collision-induced absorptions by molecular hydrogen.Comment: 33 pages, 10 figures, accepted for publication in the Astronomical
Journa
Discovery of Five New R Coronae Borealis Stars in the MACHO Galactic Bulge Database
We have identified five new R Coronae Borealis (RCB) stars in the Galactic
bulge using the MACHO Project photometry database, raising the total number of
known Galactic RCB stars to about 40. We have obtained spectra to confirm the
identifications. The fact that four out of the five newly identified RCB stars
are ``cool'' (T(eff) 6000 K) suggests
that the preponderance of warm RCB stars among the existing sample is a
selection bias. These cool RCB stars are redder and fainter than their warm
counterparts and may have been missed in surveys done with blue plates. Based
on the number of new RCB stars discovered in the MACHO bulge fields, there may
be ~250 RCB stars in the reddened "exclusion" zone toward the bulge.Comment: 16 pages, 5 figures, AJ in press High resolution versions of Figures
1 and 2 can be downloaded from
http://morpheus.phys.lsu.edu/~gclayton/figs.pdf (more typos corrected
Recommended from our members
Effects of Cerium Removal from Glass on Photovoltaic Module Performance and Stability: Preprint
Some PV manufacturers use glass without Ce, which improves light transmission but can lower long-term stability; additional light transmission in 300-340-nm range can cause delamination ~3.8x faster
Flash Mixing on the White Dwarf Cooling Curve: FUSE Observations of three He-rich sdB Stars
We present FUSE spectra of 3 He-rich sdB stars. Two of these stars,
PG1544+488 and JL87, reveal extremely strong C III lines at 977 and 1176A,
while the carbon lines are quite weak in the third star, LB1766. We have
analyzed the FUSE data using TLUSTY NLTE line-blanketed model atmospheres, and
find that PG1544+488 has a surface composition of 96% He, 2% C, and 1% N. JL87
shows a similar surface enrichment of carbon and nitrogen, but some significant
fraction of hydrogen still remains in its atmosphere. LB1766 has a surface
composition devoid of hydrogen and strongly depleted of carbon, indicating that
its surface material has undergone CN-cycle processing. We interpret these
observations with new evolutionary calculations which suggest that He-rich sdB
stars with C-rich compositions are the progeny of stars which underwent a
delayed He-core flash on the white-dwarf cooling curve. During such a flash the
interior convection zone will penetrate into the H envelope, thereby mixing the
envelope with the He- and C-rich core. Such `flash-mixed' stars will arrive on
the extreme horizontal branch (EHB) with He- and C-rich surface compositions
and will be hotter than the hottest canonical (i.e., unmixed) EHB stars. Two
types of flash mixing are possible: `deep' and `shallow', depending on whether
the H envelope is mixed deeply into the site of the He flash or only with the
outer layers of the core. Based on both their stellar parameters and surface
compositions, we suggest that PG1544+488 and JL87 are examples of `deep' and
`shallow' flash mixing, respectively. Flash mixing may therefore represent a
new evolutionary channel for producing the hottest EHB stars. However, flash
mixing cannot explain the abundance pattern in LB1766, which remains a
challenge to current evolutionary models.Comment: 32 pages, 6 figures; minor additions; to appear in Astrophys. J.,
February 200
The Formation Rate, Mass and Luminosity Functions of DA White Dwarfs from the Palomar Green Survey
Spectrophotometric observations at high signal-to-noise ratio were obtained
of a complete sample of 347 DA white dwarfs from the Palomar Green (PG) Survey.
Fits of observed Balmer lines to synthetic spectra calculated from
pure-hydrogen model atmospheres were used to obtain robust values of Teff, log
g, masses, radii, and cooling ages. The luminosity function of the sample,
weighted by 1/Vmax, was obtained and compared with other determinations. The
mass distribution of the white dwarfs is derived, after important corrections
for the radii of the white dwarfs in this magnitude-limited survey and for the
cooling time scales. The formation rate of DA white dwarfs from the PG is
estimated to be 0.6x10^(-12) pc^(-3) yr^(-1). Comparison with predictions from
a theoretical study of the white dwarf formation rate for single stars
indicates that >80% of the high mass component requires a different origin,
presumably mergers of lower mass double degenerate stars. In order to estimate
the recent formation rate of all white dwarfs in the local Galactic disk,
corrections for incompleteness of the PG, addition of the DB-DO white dwarfs,
and allowance for stars hidden by luminous binary companions had to be applied
to enhance the rate. An overall formation rate of white dwarfs recently in the
local Galactic disk of 1.15+/-0.25x10^(-12) pc^(-3) yr^(-1) is obtained. Two
recent studies of samples of nearby Galactic planetary nebulae lead to
estimates around twice as high. Difficulties in reconciling these
determinations are discussed.Comment: 73 pages, 18 figures, accepted for publication in the ApJ Supplemen
X-ray Emitting QSOs Ejected from Arp 220
Four compact ROSAT X-ray sources very close to the nearby ultraluminous
infrared galaxy Arp 220 (IC 4553) have been identified as medium-redshift QSOs.
The closest pair lying symmetrically across the galaxy have almost identical
redshifts z = 1.25, 1.26. All of the evidence suggests that these QSOs have
been ejected from Arp 220 and have large intrinsic redshifts.Comment: submitted to ApJ Letters, Figs. 1-3 included as JPE
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