328 research outputs found
Wage effects of an extra year of lower vocational education: evidence from a simultaneous change of compulsory school leaving age and program length
Cataclysmic Variables and a New Class of Faint UV Stars in the Globular Cluster NGC 6397
We present evidence that the globular cluster NGC 6397 contains two distinct
classes of centrally-concentrated UV-bright stars. Color-magnitude diagrams
constructed from U, B, V, and I data obtained with the HST/WFPC2 reveal seven
UV-bright stars fainter than the main-sequence turnoff, three of which had
previously been identified as cataclysmic variables (CVs). Lightcurves of these
stars show the characteristic ``flicker'' of CVs, as well as longer-term
variability. A fourth star is identified as a CV candidate on the basis of its
variability and UV excess. Three additional UV-bright stars show no photometric
variability and have broad-band colors characteristic of B stars. These
non-flickering UV stars are too faint to be extended horizontal branch stars.
We suggest that they could be low-mass helium white dwarfs, formed when the
evolution of a red giant is interrupted, due either to Roche-lobe overflow onto
a binary companion, or to envelope ejection following a common-envelope phase
in a tidal-capture binary. Alternatively, they could be very-low-mass
core-He-burning stars. Both the CVs and the new class of faint UV stars are
strongly concentrated toward the cluster center, to the extent that mass
segregation from 2-body relaxation alone may be unable to explain their
distribution.Comment: 11 pages plus 3 eps figures; LaTeX using aaspp4.sty; to appear in The
Astrophysical Journal Letter
Force Dependence of RF MEMS Switch Contact Heating
Contact-type RF MEMS switches have demonstrated low on-state resistance, high off-state impedance, and very large bandwidth; however, their power handling capability is low due to failure caused by contact heating. This paper examines contact heating by measuring V-I curves for contacts in gold switches. Multiphysics modeling allows extraction of contact temperature. Contacts are found to soften and self-anneal at a temperature of about 100ÂĄC, corresponding to a contact voltage of about 80 mV. Larger contact force induces a larger decrease in contact resistance during softening, suppressing contact heating. The data provide a better understanding of micro-scale contact physics, leading to design for switches for improved power-handling capability.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87263/4/Saitou88.pd
Post-common envelope binaries from SDSS - XVI. Long orbital period systems and the energy budget of CE evolution
Virtually all close compact binary stars are formed through common-envelope
(CE) evolution. It is generally accepted that during this crucial evolutionary
phase a fraction of the orbital energy is used to expel the envelope. However,
it is unclear whether additional sources of energy, such as the recombination
energy of the envelope, play an important role. Here we report the discovery of
the second and third longest orbital period post-common envelope binaries
(PCEBs) containing white dwarf (WD) primaries, i.e. SDSSJ121130.94-024954.4
(Porb = 7.818 +- 0.002 days) and SDSSJ222108.45+002927.7 (Porb = 9.588 +- 0.002
days), reconstruct their evolutionary history, and discuss the implications for
the energy budget of CE evolution. We find that, despite their long orbital
periods, the evolution of both systems can still be understood without
incorporating recombination energy, although at least small contributions of
this additional energy seem to be likely. If recombination energy significantly
contributes to the ejection of the envelope, more PCEBs with relatively long
orbital periods (Porb >~ 1-3 day) harboring massive WDs (Mwd >~ 0.8 Msun)
should exist.Comment: Accepted for publication in MNRAS. 8 pages, 6 figures and 4 table
On the Binding Energy Parameter of Common Envelope Evolution. Dependency on the Definition of the Stellar Core Boundary during Spiral-in
According to the standard picture for binary interactions, the outcome of
binaries surviving the evolution through a common envelope (CE) and spiral-in
phase is determined by the internal structure of the donor star at the onset of
the mass transfer, as well as the poorly-known efficiency parameter, eta_CE},
for the ejection of the H-envelope of the donor. In this Research Note we
discuss the bifurcation point which separates the ejected, unprocessed H-rich
material from the inner core region of the donor (the central part of the star
which will later contract to form a compact object). We demonstrate that the
exact location of this point is very important for evaluating the binding
energy parameter, lambda, which is used to determine the post-CE orbital
separation. Here we compare various methods to define the bifurcation point
(core/envelope boundary) of evolved stars with masses 4, 7, 10 and 20 M_sun. We
consider the specific nuclear energy production rate profile, the change in the
mass-density gradient (Bisscheroux 1998), the inner region containing less than
10% hydrogen, the method suggested by Han et al. (1994) and the entropy
profile. We also calculated effective polytropic index profiles. The entropy
profile method measures the convective boundary (at the onset of flatness in
the specific entropy) which is not equivalent to the core boundary for RGB
stars. Hence, this method is not applicable for RGB stars, unless the actual
bifurcation point of a CE is located at the bottom of the outer convection zone
(resulting in larger values of lambda and larger post-CE orbital separations).
On the AGB, where highly degenerate and condensed cores are formed, we find
good agreement between the various methods, except for massive (20 M_sun)
stars.Comment: 4 pages, 1 figure, A&A in pres
Angular Resolution of the LISA Gravitational Wave Detector
We calculate the angular resolution of the planned LISA detector, a
space-based laser interferometer for measuring low-frequency gravitational
waves from galactic and extragalactic sources. LISA is not a pointed
instrument; it is an all-sky monitor with a quadrupolar beam pattern. LISA will
measure simultaneously both polarization components of incoming gravitational
waves, so the data will consist of two time series. All physical properties of
the source, including its position, must be extracted from these time series.
LISA's angular resolution is therefore not a fixed quantity, but rather depends
on the type of signal and on how much other information must be extracted.
Information about the source position will be encoded in the measured signal in
three ways: 1) through the relative amplitudes and phases of the two
polarization components, 2) through the periodic Doppler shift imposed on the
signal by the detector's motion around the Sun, and 3) through the further
modulation of the signal caused by the detector's time-varying orientation. We
derive the basic formulae required to calculate the LISA's angular resolution
for a given source. We then evaluate for
two sources of particular interest: monchromatic sources and mergers of
supermassive black holes. For these two types of sources, we calculate (in the
high signal-to-noise approximation) the full variance-covariance matrix, which
gives the accuracy to which all source parameters can be measured. Since our
results on LISA's angular resolution depend mainly on gross features of the
detector geometry, orbit, and noise curve, we expect these results to be fairly
insensitive to modest changes in detector design that may occur between now and
launch. We also expect that our calculations could be easily modified to apply
to a modified design.Comment: 15 pages, 5 figures, RevTex 3.0 fil
RR Lyrae stars in four globular clusters in the Fornax dwarf galaxy
(Abridged) We have surveyed four globular clusters in the Fornax dwarf galaxy
for RR Lyrae stars, using archival HST observations. We identify 197 new RR
Lyrae stars in these four clusters. Despite the short observational baseline,
we derive periods, light-curves, and photometric parameters for each. The
Fornax clusters have exceptionally large RR Lyrae specific frequencies compared
with the Galactic globular clusters. Furthermore, the Fornax cluster RR Lyrae
stars are unusual in that their characteristics are intermediate between the
two Galactic Oosterhoff groups. In this respect the Fornax clusters are similar
to the field populations in several dwarf galaxies. We revise previous
measurements of the HB morphology in each cluster. The Fornax clusters closely
resemble the ``young'' Galactic halo population defined by Zinn. The existence
of the second parameter effect among the Fornax clusters is also confirmed.
Finally, we determine foreground reddening and distance estimates for each
cluster. We find a mean distance modulus to Fornax of (m-M)_0 = 20.66 +/- 0.03
(random) +/- 0.15 (systematic). Our measurements are consistent with a line of
sight depth of 8-10 kpc for this galaxy, matching its projected dimensions, and
incompatible with tidal model explanations for the observed high velocity
dispersions in many dSph galaxies. Dark matter dominance is suggested.Comment: 26 pages, 6 figures. Accepted for publication in MNRAS. Table 2 and
Figure 2 will only be available in the electronic version. On-line data will
soon be available at http://www.ast.cam.ac.uk/STELLARPOPS/Fornax_RRlyr
Subluminous B Stars and Progenitors of Helium Core White Dwarfs
Subluminous B (sdB) stars can result from stable Roche lobe overflow or common envelope ejection in close binary systems. The companions are either low mass main sequence stars or white dwarfs. We discuss mass determinations for sdB stars from such systems and report the discovery of the bright nearby sdB star, HD 188112, to be a close binary system of unusually low mass (0.23 solar masses). From the mass function it is evident that the companion is a massive compact object of at least 0.72 solar masses. The mass of the sdB HD 188112 is too low for the star to evolve into a C/O white dwarf, but it probably is a progenitor for a helium core white dwarf
Type Ia supernova SN 2003du: optical observations
UBVRI photometry and optical spectra of type Ia supernova SN 2003du obtained
at the Indian Astronomical Observatory for nearly a year since discovery are
presented.
The apparent magnitude at maximum was B=13.53 +/- 0.02 mag, and the colour
(B-V) = -0.08 +/- 0.03 mag. The luminosity decline rate, Delta(m_{15}(B)) =
1.04 +/- 0.04 mag indicates an absolute B magnitude at maximum of M_B = -19.34
+/- 0.3 mag and the distance modulus to the parent galaxy as mu=32.89 +/-
0.4.The light curve shapes are similar, though not identical, to those of SNe
1998bu and 1990N, both of which had luminosity decline rates similar to that of
SN 2003du and occurred in spiral galaxies. The peak bolometric luminosity
indicates that 0.9 Msun mass of 56Ni was ejected by the supernova. The spectral
evolution and the evolution of the Si II and Ca II absorption velocities
closely follows that of SN 1998bu, and in general, is within the scatter of the
velocities observed in normal type Ia supernovae.
The spectroscopic and photometric behaviour of SN 2003du is quite typical for
SNe Ia in spirals.
A high velocity absorption component in the Ca II (H & K) and IR-triplet
features, with absorption velocities of ~20,000 km/s and ~22,000 km/s
respectively, is detected in the pre-maximum spectra of days -11 and -7.Comment: 10 pages, 10 figures; Accepted for publication in A&
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