1,128 research outputs found
Confirmation of Eclipses in KPD 0422+5421, A Binary Containing a White Dwarf and a Subdwarf B Star
We report additional photometric CCD observations of KPD 0422+5421, a binary
with an orbital period of 2.16 hours which contains a subdwarf B star (sdB) and
a white dwarf. There are two main results of this work. First, the light curve
of KPD 0422+5421 contains two distinct periodic signals, the 2.16 hour
ellipsoidal modulation discovered by Koen, Orosz, & Wade (1998) and an
additional modulation at 7.8 hours. This 7.8 hour modulation is clearly not
sinusoidal: the rise time is about 0.25 in phase, whereas the decay time is
0.75 in phase. Its amplitude is roughly half of the amplitude of the
ellipsoidal modulation. Second, after the 7.8 hour modulation is removed, the
light curve folded on the orbital period clearly shows the signature of the
transit of the white dwarf across the face of the sdB star and the signature of
the occultation of the white dwarf by the sdB star. We used the Wilson-Devinney
code to model the light curve to obtain the inclination, the mass ratio, and
the Omega potentials, and a Monte Carlo code to compute confidence limits on
interesting system parameters. We find component masses of M_sdB = 0.36 +/-
0.16 solar masses and M_WD = 0.47 +/- 0.16 solar masses (M_total = 0.86 +/-
0.35 solar masses, 68 per cent confidence limits). If we impose an additional
constraint and require the computed mass and radius of the white dwarf to be
consistent with a theoretical mass-radius relation, we find M_sdB = 0.511
+0.047 -0.050 solar masses and M_WD = 0.526 +0.033 -0.030 solar masses (68 per
cent confidence limits). In this case the total mass of the system is less than
1.4 solar masses at the 99.99 per cent confidence level. We briefly discuss
possible interpretations of the 7.8 hour modulation and the importance of KPD
0422+5421 as a member of a rare class of evolved binaries.Comment: 11 pages, 7 figures, to appear in MNRAS, LaTeX, uses mn.st
2MASS J05162881+2607387: A New Low-Mass Double-Lined Eclipsing Binary
We show that the star known as 2MASS J05162881+2607387 (hereafter J0516) is a
double-lined eclipsing binary with nearly identical low-mass components. The
spectroscopic elements derived from 18 spectra obtained with the High
Resolution Spectrograph on the Hobby-Eberly Telescope during the Fall of 2005
are K_1=88.45 +/- 0.48 km/s and K_2=90.43 +/- 0.60 km/s, resulting in a mass
ratio of$q=K_1/K_2 = 0.978 +/- 0.018 and minimum masses of M_1 sin^{3}i=0.775
+/- 0.016 solar masses and M_2 sin^{3}i=0.759 +/- 0.012 solar masses,
respectively. We have extensive differential photometry of J0516 obtained over
several nights between 2004 January-March (epoch 1) and 2004 October-2005
January plus 2006 January (epoch 2) using the 1m telescope at the Mount Laguna
Observatory. The source was roughly 0.1 mag brighter in all three bandpasses
during epoch 1 when compared to epoch 2. Also, phased light curves from epoch 1
show considerable out-of-eclipse variability, presumably due to bright spots on
one or both stars. In contrast, the phased light curves from epoch 2 show
little out-of-eclipse variability. The light curves from epoch 2 and the radial
velocity curves were analyzed using our ELC code with updated model atmospheres
for low-mass stars. We find the following: M_1=0.787 +/- 0.012 solar masses,
R_1=0.788 +/- 0.015 solar radii, M_2=0.770 +/- 0.009 solar masses, and
R_2=0.817 +/- 0.010 solar radii. The stars in J0516 have radii that are
significantly larger than model predictions for their masses, similar to what
is seen in a handful of other well-studied low-mass double-lined eclipsing
binaries. We compiled all recent mass and radius determinations from low-mass
binaries and determine an empirical mass-radius relation of the form R = 0.0324
+ 0.9343M + 0.0374M^2, where the quantities are in solar units.Comment: 16 pages, 10 figures (Figure 1 has degraded quality), to appear in
Ap
OGLE observations of four X-ray binary pulsars in the SMC
This paper presents analysis and interpretation of OGLE photometric data of
four X-ray binary pulsar systems in the Small Magellanic Cloud: 1WGA
J0054.9-7226, RX J0050.7-7316, RX J0049.1-7250, and 1SAX J0103.2-7209. In each
case, the probable optical counterpart is identified on the basis of its
optical colours. In the case of RX J0050.7-7316 the regular modulation of its
optical light curve appears to reveal an ellipsoidal modulation with a period
of 1.416 days. Using reasonable masses for the neutron star and the B star, we
show that the amplitude and relative depths of the minima of the I-band light
curve of RX J0050.7-7316 can be matched with an ellipsoidal model where the B
star nearly fills its Roche lobe. For mass ratios in the range of 0.12 to 0.20,
the corresponding best-fitting inclinations are about 55 degrees or larger. The
neutron star would be eclipsed by the B star at inclinations larger than 60
degrees for this particular mass ratio range. Thus RX J0050.7-7316 is a good
candidate system for further study. In particular, we would need additional
photometry in several colours, and most importantly, radial velocity data for
the B star before we could draw more quantitative conclusions about the
component masses
KPD 0422+5421: A New Short Period Subdwarf B/White Dwarf Binary
The sdB star KPD 0422+5421 was discovered to be a single-lined spectroscopic
binary with a period of P=0.0901795 +/- (3\times 10^{-7}) days (2 hours, 10
minutes). The U and B light curves display an ellipsoidal modulation with
amplitudes of about 0.02 magnitudes. The sdB star contributes nearly all of the
observed flux. This and the absence of any reflection effect suggest that the
unseen companion star is small (i.e. R_comp ~ 0.01 solar radii) and therefore
degenerate. We modeled the U and B light curves and derived i = 78.05 +/- 0.50
degrees and a mass ratio of q = M_comp/M_sdB = 0.87 +/- 0.15. The sdB star
fills 69% of its Roche lobe. These quantities may be combined with the mass
function of the companion (f(M) = 0.126 +/- 0.028 solar masses) to derive M_sdB
= 0.72 +/- 0.26 solar masses and M_comp = 0.62 +/- 0.18 solar masses. We used
model spectra to derive the effective temperature, surface gravity, and helium
abundance of the sdB star. We found T_eff = 25,000 +/- 1500K, log g = 5.4 +/-
0.1, and [He/H] = -1.0. With a period of 2 hours and 10 minutes, KPD 0422+5421
has one of the shortest known orbital periods of a detached binary. This system
is also one of only a few known binaries which contain a subdwarf B star and a
white dwarf. Thus KPD 0422+5421 represents a relatively unobserved, and
short-lived, stage of binary star evolution.Comment: 9 pages, 8 figures, to appear in MNRAS, LaTeX, uses mn.st
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