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 100¡C, 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 $\Delta \Omega_S$ for a given source. We then evaluate $\Delta \Omega_S$ 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&