162 research outputs found
Second generation planet formation in NN Serpentis?
In this paper, we study the general impact of stellar mass-ejection events in
planetary orbits in post-common envelope binaries with circumbinary planets
like those around NN Serpentis. We discuss a set of simple equations that
determine upper and lower limits for orbital expansion and investigate the
effect of initial eccentricity. We deduce the range of possible semi-major axes
and initial eccentricity values of the planets prior to the common-envelope
event. In addition to spherically-symmetric mass-ejection events, we consider
planetary dynamics under the influence of an expanding disk. In order to have
survived, we suggest that the present planets in NN Ser must have had
semi-major axes AU and high eccentricity values which is
in conflict with current observations. Consequently, we argue that these
planets were not formed together with their hosting stellar system, but rather
originated from the fraction of matter of the envelope that remained bound to
the binary. According to the cooling age of the white dwarf primary of
yr, the planets around NN Ser might be the youngest known so far and open up a
wide range of further study of second generation planet formation.Comment: 4 pages, 2 figure
Evidence for an oscillation of the magnetic axis of the white dwarf in the polar DP Leonis
From 1979 to 2001, the magnetic axis of the white dwarf in the polar DP Leo
slowly rotated by 50 deg in azimuth, possibly indicating a small asynchronism
between the rotational and orbital periods of the magnetic white dwarf. We have
obtained phase-resolved orbital light curves between 2009 and 2013, which show
that this trend has not continued in recent years. Our data are consistent with
the theoretically predicted oscillation of the magnetic axis of the white dwarf
about an equilibrium orientation, which is defined by the competition between
the accretion torque and the magnetostatic interaction of the primary and
secondary star. Our data indicate an oscillation period of ~60 yr, an amplitude
of about 25 deg, and an equilibrium orientation leading the connecting line of
the two stars by about 7 deg.Comment: Accepted by A&
Evidence for a T Tauri Phase in Young Brown Dwarfs
As part of a multi-faceted program to investigate the origin and early
evolution of sub-stellar objects, we present high-resolution Keck optical
spectra of 14 very low mass sources in the IC 348 young cluster and the Taurus
star-forming cloud. All of our targets, which span a range of spectral types
from M5 to M8, exhibit moderate to very strong H emission. In half of
the IC 348 objects, the H profiles are broad and asymmetric, indicative
of on-going accretion. Of these, IC348-355 (M8) is the lowest mass object to
date to show accretion-like H. Three of our ~M6 IC 348 targets with
broad H also harbor broad OI (8446\AA) and CaII (8662\AA) emission, and
one shows broad HeI (6678\AA) emission; these features are usually seen in
strongly accreting classical T Tauri stars. We find that in very low mass
accretors, the H profile may be somewhat narrower than that in higher
mass stars. We propose that low accretion rates combined with small infall
velocities at very low masses can conspire to produce this effect. In the
non-accretors in our sample, H emission is commensurate with, or higher
than, saturated levels in field M dwarfs of similar spectral type. Our results
constitute the most compelling evidence to date that young brown dwarfs undergo
a T Tauri-like accretion phase similar to that in stars. This is consistent
with a common origin for most low-mass stars, brown dwarfs and isolated
planetary mass objects.Comment: to appear in The Astrophysical Journa
The spectral type of CHS7797 - an intriguing very low mass periodic variable in the Orion Nebula Cluster
We present the spectroscopic characterization of the unusual high-amplitude
very low mass pre-main-sequence periodic variable CHS7797. This study is based
on optical medium-resolution (R=2200) spectroscopy in the 6450-8600 A range,
carried out with GMOS-GEMINI-S in March 2011. Observations of CHS7797 have been
carried out at two distinct phases of the 17.8d period, namely at maximum and
four days before maximum. Four different spectral indices were used for the
spectral classification at these two phases, all of them well-suited for
spectral classification of young and obscured late M dwarfs. In addition, the
gravity-sensitive NaI (8183/8195 A) and KI (7665/7699 A) doublet lines were
used to confirm the young age of CHS7797. From the spectrum obtained at maximum
light we derived a spectral type (SpT) of M6.05, while for the spectrum taken
four days before maximum the derived SpT is M5.75. The derived SpTs confirm
that CHS7797 has a mass in the stellar-substellar boundary mass range. In
addition, the small differences in the derived SpTs at the two observed phases
may provide indirect hints that CHS7797 is a binary system of similar mass
components surrounded by a tilted circumbinary disk, a system similar to KH15D.Comment: 6 pages, accepted for publication A&
The eclipsing post-common envelope binary CSS21055: a white dwarf with a probable brown-dwarf companion
We report photometric observations of the eclipsing close binary CSS21055
(SDSS J141126+200911) that strongly suggest that the companion to the
carbon-oxygen white dwarf is a brown dwarf with a mass between 0.030 and 0.074
Msun. The measured orbital period is 121.73min and the totality of the eclipse
lasts 125s. If confirmed, CSS21055 would be the first detached eclipsing WD+BD
binary. Spectroscopy in the eclipse could provide information about the
companion's evolutionary state and atmospheric structure.Comment: 6 pages, 5 figure
Nonlinear Outcome of Gravitational Instability in Disks with Realistic Cooling
We consider the nonlinear outcome of gravitational instability in optically
thick disks with a realistic cooling function. We use a numerical model that is
local, razor-thin, and unmagnetized. External illumination is ignored. Cooling
is calculated from a one-zone model using analytic fits to low temperature
Rosseland mean opacities. The model has two parameters: the initial surface
density Sigma_0 and the rotation frequency Omega. We survey the parameter space
and find: (1) The disk fragments when t_c,eff Omega = 1, where t_c,eff is an
effective cooling time defined as the average internal energy of the model
divided by the average cooling rate. This is consistent with earlier results
that used a simplified cooling function. (2) The initial cooling time t_c0 or a
uniform disk with Q = 1 can differ by orders of magnitude from t_c,eff in the
nonlinear outcome. The difference is caused by sharp variations in the opacity
with temperature. The condition t_c0 Omega = 1 therefore does not necessarily
indicate where fragmentation will occur. (3) The largest difference between
t_c,eff and t_c0 is near the opacity gap, where dust is absent and hydrogen is
largely molecular. (4) In the limit of strong illumination the disk is
isothermal; we find that an isothermal version of our model fragments for Q <
1.4. Finally, we discuss some physical processes not included in our model, and
find that most are likely to make disks more susceptible to fragmentation. We
conclude that disks with t_c,eff Omega < 1 do not exist.Comment: 30 pages, 12 figure
A long-term optical and X-ray ephemeris of the polar EK Ursae Majoris
We searched for long-term period changes in the polar EK UMa using new
optical data and archival X-ray/EUV data. An optical ephemeris was derived from
data taken remotely with the MONET/N telescope and compared with the X-ray
ephemeris based on Einstein, Rosat, and EUVE data. A three-parameter fit to the
combined data sets yields the epoch, the period, and the phase offset between
the optical minima and the X-ray absorption dips. An added quadratic term is
insignificant and sets a limit to the period change. The derived linear
ephemeris is valid over 30 years and the common optical and X-ray period is
P=0.0795440225(24) days. There is no evidence of long-term O-C variations or a
period change over the past 17 years Delta P = -0.14+-0.50 ms. We suggest that
the observed period is the orbital period and that the system is tightly
synchronized. The limit on Delta P and the phase constancy of the bright part
of the light curve indicate that O-C variations of the type seen in the polars
DP Leo and HU Aqr or the pre-CV NN Ser do not seem to occur in EK UMa. The
X-ray dips lag the optical minima by 9.5+-0.7 deg in azimuth, providing some
insight into the accretion geometry.Comment: 4 pages, 2 Postscript figures, accepted for publication in Astronomy
& Astrophysic
Very Low Mass Stars and Brown Dwarfs in Taurus-Auriga
We present high resolution optical spectra obtained with the HIRES
spectrograph on the Keck I telescope of low mass T Tauri stars and brown dwarfs
(LMTTs) in Taurus-Auriga. Of particular interest is the previously classified
"continuum T Tauri star" GM Tau, which has a spectral type of M6.5 and a mass
just below the stellar/substellar boundary. None of the LMTTs in Taurus are
rapidly rotating (vsini < 30 km/s), unlike low mass objects in Orion. Many of
the slowly rotating, non-accreting stars and brown dwarfs exhibit prominent
H-alpha emission (EWs of 3 - 36 A), indicative of active chromospheres. We
demonstrate empirically that the full-width at 10% of the H-alpha emission
profile peak is a more practical and possibly more accurate indicator of
accretion than either the equivalent width of H-alpha or optical veiling:
10%-widths > 270 km/s are classical T Tauri stars (i.e. accreting), independent
of stellar spectral type. Although LMTTs can have accretion rates comparable to
that of more typical, higher-mass T Tauri stars (e.g. K7-M0), the average mass
accretion rate appears to decrease with decreasing mass. The diminished
frequency of accretion disks for LMTTs, in conjunction with their lower, on
average, mass accretion rates, implies that they are formed with less massive
disks than higher-mass T Tauri stars. The radial velocities, circumstellar
properties and known binaries do not support the suggestion that many of the
lowest mass members of Taurus have been ejected from higher stellar density
regions within the cloud. Instead, LMTTs appear to have formed and are evolving
in the same way as higher-mass T Tauri stars, but with smaller disks and
shorter disk lifetimes.Comment: 27 pages, plus 8 figures, accepted for publication in Ap
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