26 research outputs found
A spectroscopic search for faint secondaries in cataclysmic variables
The secondary in cataclysmic variables (CV's) is usually detected by
cross-correlation of the CV spectrum with that of a K or M dwarf template, to
produce a radial velocity curve. Although this method has demonstrated its
power, it has its limits in the case of noisy spectra, such as are found when
the secondary is faint. A method of co-adding spectra, called skew mapping, has
been proposed in the past. Gradually, examples of its application are being
published. Nonetheless, so far no journal article has described the technique
in detail. To answer this need, this paper explores in detail the capabilities
of skew mapping when determining the amplitude of the radial velocity for faint
secondaries. It demonstrates the method's power over techniques that are more
conventional, when the signal-to-noise (s/n) ratio is poor. The paper suggests
an approach to assessing the quality of results. This leads in the case of the
investigated objects to a first tier of results, where we find K2=127+-23 km/s
for SY Cnc, K2=144+-18 km/s for RW Sex, and K2=262+-14 km/s for UX UMa. These
we believe to be the first direct determinations of K2 for these objects.
Furthermore, we also obtain K2=263+-30 km/s for RW Tri, close to a skew mapping
result obtained elsewhere. In the first three cases, we use these results to
derive the mass of the white dwarf companion. A second tier of results includes
UU Aqr, EX Hya, and LX Ser, for which we propose more tentative values of K2.
Clear failures of the method are also discussed (EF Eri, VV Pup, SW Sex).Comment: 23 pages, 22 figures, 10 tables; translated to pdf from a Word file -
no TeX version available. Accepted for publication in MNRA
The birthplace and age of the isolated neutron star RX J1856.5-3754
X-ray observations unveiled various types of radio-silent Isolated Neutron
Stars (INSs), phenomenologically very diverse, e.g. the Myr old X-ray Dim INS
(XDINSs) and the kyr old magnetars. Although their phenomenology is much
diverse, the similar periods (P=2--10 s) and magnetic fields (~10^{14} G)
suggest that XDINSs are evolved magnetars, possibly born from similar
populations of supermassive stars. One way to test this hypothesis is to
identify their parental star clusters by extrapolating backward the neutron
star velocity vector in the Galactic potential. By using the information on the
age and space velocity of the XDINS RX J1856.5-3754, we computed backwards its
orbit in the Galactic potential and searched for its parental stellar cluster
by means of a closest approach criterion. We found a very likely association
with the Upper Scorpius OB association, for a neutron star age of 0.42+/-0.08
Myr, a radial velocity V_r^NS =67+/- 13$ km s^{-1}, and a present-time
parallactic distance d_\pi^NS = 123^{+11}_{-15} pc. Our result confirms that
the "true" neutron star age is much lower than the spin-down age (tau_{sd}=3.8
Myrs), and is in good agreement with the cooling age, as computed within
standard cooling scenarios. The mismatch between the spin-down and the
dynamical/cooling age would require either an anomalously large breaking index
(n~20) or a decaying magnetic field with initial value B_0 ~ 10^{14} G.
Unfortunately, owing to the uncertainty on the age of the Upper Scorpius OB
association and the masses of its members we cannot yet draw firm conclusions
on the estimated mass of the RX J1856.5-3754 progenitor.Comment: 6 pages, accepted for publication on Monthly Notices of the Royal
Astronomical Societ
A kinematic study of Open Clusters: implications for their origin
The Galactic population of open clusters provides an insight into star
formation in the Galaxy. The open cluster catalogue by Dias et al.(2002b) is a
rich source of data, including kinematic information. This large sample made it
possible to carry out a systematic analysis of 481 open cluster orbits, using
parameters based on orbit eccentricity and separation from the Galactic plane.
These two parameters may be indicative of origin, and we find them to be
correlated. We also find them to be correlated with metallicity, another
parameter suggested elsewhere to be a marker for origin in that high values of
any of these two parameters generally indicates a low metallicity ([Fe/H]
Solar0.2 dex). The resulting analysis points to four open clusters in the
catalogue being of extra-Galactic origin by impact of high velocity cloud on
the disk: Berkeley21, 32, 99, and Melotte66, with a possible further four due
to this origin (NGC2158, 2420, 7789, IC1311). A further three may be due to
Galactic globular cluster impact on the disk i.e of internal Galactic origin
(NGC6791, 1817, and 7044).Comment: 14 pages, 816 figures, accepted for publication in MNRAS 14-May-201
The masses of the cataclysmic variables AC Cancri and V363 Aurigae
We present time-resolved spectroscopy and photometry of the double-lined eclipsing cataclysmic variables AC Cnc and V363 Aur (= Lanning 10). There is evidence of irradiation on the inner hemisphere of the secondary star in both systems, which we correct for using a model that reproduces the observations remarkably well. We find the radial velocity of the secondary star in AC Cnc to be KR= 176 ± 3 km sâ1 and its rotational velocity to be v sin i= 135 ± 3 km sâ1. From these parameters we obtain masses of M1= 0.76 ± 0.03 Mâ for the white-dwarf primary and M2= 0.77 ± 0.05 Mâ for the K2 ± 1V secondary star, giving a mass ratio of q= 1.02 ± 0.04. We measure the radial and rotational velocities of the G7 ± 2V secondary star in V363 Aur to be KR= 168 ± 5 km sâ1 and v sin i= 143 ± 5 km sâ1, respectively. The component masses of V363 Aur are M1= 0.90 ± 0.06 Mâ and M2= 1.06 ± 0.11 Mâ, giving a mass ratio of q= 1.17 ± 0.07. The mass ratios for AC Cnc and V363 Aur fall within the theoretical limits for dynamically and thermally stable mass transfer. Both systems are similar to the SW Sex stars, exhibiting single-peaked emission lines with transient absorption features, high-velocity S-wave components and phase-offsets in their radial-velocity curves. The Balmer lines in V363 Aur show a rapid increase in flux around phase 0 followed by a rapid decrease, which we attribute to the eclipse of an optically thick region at the centre of the disc. This model could also account for the behaviour of other SW Sex stars where the Balmer lines show only a shallow eclipse compared to the continuum
The component masses of the cataclysmic variable V347 Puppis
We present time-resolved spectroscopy and photometry of the double-lined eclipsing cataclysmic variable V347 Pup (=LB 1800). There is evidence of irradiation on the inner hemisphere of the secondary star, which we correct for using a model to give a secondary-star radial velocity of KR= 198 ± 5 km sâ1. The rotational velocity of the secondary star in V347 Pup is found to be v sin i= 131 ± 5 km sâ1 and the system inclination is i= 840 ± 23. From these parameters we obtain masses of M1= 0.63 ± 0.04 Mâ for the white dwarf primary and M2= 0.52 ± 0.06 Mâ for the M0.5V secondary star, giving a mass ratio of q= 0.83 ± 0.05. On the basis of the component masses, and the spectral type and radius of the secondary star in V347 Pup, we find tentative evidence for an evolved companion. V347 Pup shows many of the characteristics of the SW Sex stars, exhibiting single-peaked emission lines, high-velocity S-wave components and phase-offsets in the radial velocity curve. We find spiral arms in the accretion disc of V347 Pup and measure the disc radius to be close to the maximum allowed in a pressureless disc
Outbursts of EX Hydrae Revisited
We present optical spectroscopy of EX Hya during its 1991 outburst. This
outburst is characterised by strong irradiation of the front face of the
secondary star by the white dwarf, an overflowing stream which is seen strongly
in HeII and by a dip in the light curves, which extends from 0.1-0.6 in the
binary and spin phases. Strong irradiation of the accretion curtain and that of
the inner regions of the disc led to strong emission of HeII and to the
suppression of the Hg and Hb emission.
Disc overflow was observed in quiescence in earlier studies, where the
overflow stream material was modulated at high velocities close to 1000 km/s.
In outburst, the overflowing material is modulated at even higher velocities
(~1500 km/s). These are streaming velocities down the field lines close to the
white dwarf. Evidence for material collecting near the outer edge of the disc
and corotating with the accretion curtain was observed. In decline, this
material and the accretion curtain obscured almost all the emission near binary
phase 0.4, causing a dip. The dip minimum nearly corresponds with spin pulse
minimum. This has provided additional evidence for an extended accretion
curtain, and for the corotation of material with the accretion curtain at the
outer edge of the disc. From these observations we suggest that a mechanism
similar to that of Spruit & Taam, where outbursts result due to the storage and
release of matter outside the magnetosphere, triggers the outbursts of EX Hya.
This is followed by the irradiation of the secondary star due to accretion
induced radiation.Comment: 12 pages, 14 figures, 1 table. Figures 6, 7, 8 and 11 at low
resolution. Paper accepted by the Monthly Notices of the Royal Astronomical
Societ