37 research outputs found
MOCCA-SURVEY database I. Accreting white dwarf binary systems in globular clusters -- IV. cataclysmic variables -- properties of bright and faint populations
We investigate here populations of cataclysmic variables (CVs) in a set of
288 globular cluster (GC) models evolved with the MOCCA code. This is by far
the largest sample of GC models ever analysed with respect to CVs. Contrary to
what has been argued for a long time, we found that dynamical destruction of
primordial CV progenitors is much stronger in GCs than dynamical formation of
CVs, and that dynamically formed CVs and CVs formed under no/weak influence of
dynamics have similar white dwarf mass distributions. In addition, we found
that, on average, the detectable CV population is predominantly composed of CVs
formed via typical common envelope phase (CEP) ( per cent), that
only per cent of all CVs in a GC is likely to be detectable, and
that core-collapsed models tend to have higher fractions of bright CVs than
non-core-collapsed ones. We also consistently show, for the first time, that
the properties of bright and faint CVs can be understood by means of the pre-CV
and CV formation rates, their properties at their formation times and cluster
half-mass relaxation times. Finally, we show that models following the initial
binary population proposed by Kroupa and set with low CEP efficiency better
reproduce the observed amount of CVs and CV candidates in NGC 6397, NGC 6752
and 47 Tuc. To progress with comparisons, the essential next step is to
properly characterize the candidates as CVs (e.g. by obtaining orbital periods
and mass ratios).Comment: 18 pages, 13 figures; accepted for publication in MNRA
The continuous cadence Roman Galactic Bulge survey
Galactic binaries with orbital periods less than 1 hour are strong
gravitational wave sources in the mHz regime, ideal for the Laser
Interferometer Space Antenna (LISA). At least several hundred, maybe up to a
thousand of those binaries are predicted to be sufficiently bright in
electromagnetic wavebands to allow detection in both the electromagnetic and
the gravitational bands allowing us to perform multi-messenger studies on a
statistically significant sample. Theory predicts that a large number of these
sources will be located in the Galactic Plane and in particular towards the
Galactic Bulge region. Some of these tight binaries may host sub-stellar
tertiaries. In this white paper we propose an observing strategy for the
Galactic Bulge Time Domain Survey which would use the unique observing
capabilities of the Nancy Grace Roman Space telescope to discover and study
several 10s of new strong LISA gravitational sources as well as exoplanet
candidates around compact white dwarf binaries and other short period variables
such as flaring stars, compact pulsators and rotators.Comment: 5 pages, 1 figure; Submitted to the NASA Roman Core Community Surveys
White Paper Cal
Strongly magnetized accretion in ultracompact binary systems
AM CVn systems are binary star systems with orbital periods less than 70 minutes in which a white dwarf accretes matter from a companion star, which must be either a stripped helium burning star, or a white dwarf of lower mass than the accretor. Here, we present the discoveries of two of these systems in which there is mass transfer from the lighter white dwarf or helium star onto a strongly magnetized heavier white dwarf. These represent the first clear example of magnetized accretion in ultracompact binaries. These systems, along with similar systems that are slightly more widely separated, and that have not started to transfer mass yet, are expected to be the primary source of gravitational waves to be detected by space-based gravitational wave observatories. The presence of strong magnetic fields can substantially affect both the evolution of the binaries, and also the particular wave forms of the gravitational waves themselves, and understanding these magnetic effects is vital for understanding what to expect from the Laser Interferometer Space Antenna
Strongly magnetized accretion in two ultracompact binary systems
We present the discoveries of two of AM CVn systems, Gaia14aae and
SDSS~J080449.49+161624.8, which show X-ray pulsations at their orbital periods,
indicative of magnetically collimated accretion. Both also show indications of
higher rates of mass transfer relative to the expectations from binary
evolution driven purely by gravitational radiation, based on existing optical
data for Gaia14aae, which show a hotter white dwarf temperature than expected
from standard evolutionary models, and X-ray data for SDSS~J080449.49+161624.8
which show a luminosity 10-100 times higher than those for other AM~CVn at
similar orbital periods. The higher mass transfer rates could be driven by
magnetic braking from the disk wind interacting with the magnetosphere of the
tidally locked accretor. We discuss implications of this additional angular
momentum transport mechanism for evolution and gravitational wave detectability
of AM CVn objects.Comment: 5 pages, 3 figures, accepted to MNRAS Letter
A candidate magnetic helium core white dwarf in the globular cluster NGC 6397
We report a peculiar variable blue star in the globular cluster NGC 6397,
using Hubble Space Telescope optical imaging. Its position in the
colour-magnitude diagrams, and its spectrum, are consistent with this star
being a helium core white dwarf (He WD) in a binary system. The optical light
curve shows a periodicity at 18.5 hours. We argue that this periodicity is due
to the rotation of the WD and possibly due to magnetic spots on the surface of
the WD. This would make this object the first candidate magnetic He WD in any
globular cluster (GC), and the first candidate magnetic WD in a detached binary
system in any GC and one of the few He WDs with a known rotation period and of
magnetic nature. Another possibility is that this system is a He WD in a binary
system with another WD or another degenerate object, which would make this
object one of the few candidate non-accreting double degenerate binaries in any
GC.Comment: Accepted for publication in MNRA
X-Ray Observation of the Roche-lobe-filling White Dwarf plus Hot Subdwarf System ZTF J213056.71+442046.5
ZTF J213056.71+442046.5 is the prototype of a small class of recently discovered compact binaries composed of a white dwarf and a hot subdwarf that fills its Roche lobe. Its orbital period of only 39 minutes is the shortest known for the objects in this class. Evidence for a high orbital inclination (i = 86°) and for the presence of an accretion disk has been inferred from a detailed modeling of its optical photometric and spectroscopic data. We report the results of an XMM-Newton observation carried out on 2021 January 7. ZTF J213056.71+442046.5 was clearly detected by the Optical Monitor, which showed a periodic variability in the UV band (200–400 nm), with a light curve similar to that seen at longer wavelengths. Despite accretion on the white dwarf at an estimated rate of the order of 10−9 M⊙ yr−1, no X-rays were detected with the EPIC instrument, with a limit of ∼1030 erg s−1 on the 0.2–12 keV luminosity. We discuss possible explanations for the lack of a strong X-ray emission from this system
Rubin Observatory LSST Transients and Variable Stars Roadmap
The Vera C. Rubin Legacy Survey of Space and Time holds the potential to revolutionize time domain astrophysics, reaching completely unexplored areas of the Universe and mapping variability time scales from minutes to a decade. To prepare to maximize the potential of the Rubin LSST data for the exploration of the transient and variable Universe, one of the four pillars of Rubin LSST science, the Transient and Variable Stars Science Collaboration, one of the eight Rubin LSST Science Collaborations, has identified research areas of interest and requirements, and paths to enable them. While our roadmap is ever-evolving, this document represents a snapshot of our plans and preparatory work in the final years and months leading up to the survey\u27s first light