20 research outputs found
The difficulty of measuring the local dark matter density
The analysis of the vertical velocity dispersion of disc stars is the most
direct astronomical means of estimating the local dark matter density,
. Current estimates based on the mid-plane dynamic density use a
local baryonic correction that ignores the non-local effects of spiral
structure and significantly underestimates the amount of dynamically relevant
gas; the additional gas plus the remaining uncertainties make it practically
impossible to measure from mid-plane kinematics alone. The sampling
of inhomogeneous tracer populations with different scale-heights and
scale-lengths results in a systematic increase in the observed dispersion
gradients and changes in the nominal density distributions that, if not
properly considered, can be misinterpreted as a sign of more dark matter. If
the disc gravity is modelled using an infinite disc, the local variation in the
vertical gravity due to the globally exponential disc components results in an
underestimation of the baryonic contribution by as much as ~40% Given only the
assumptions of stationarity, an axially and vertically symmetric disc, doubly
exponential tracer and mass-component density profiles, a phenomenologically
justified model for the cross-dispersion component , and a
realistic model for , it is possible to solve the full vertical Jeans
equation analytically for the vertical dispersion and hence
test the robustness of previous attempts at measuring . When the
model parameters are estimated from SEGUE G dwarf star data, it is still not
possible to explain the difference in behaviour seen in the simple thick- and
thin-disc datasets reported by Buedenbender et al.. Rather than being a
fundamental problem with the kinematical model, this effect appears to be a
further sign of the difficulty of defining and handling kinematically
homogeneous tracer populations.Comment: 11 pages, 3 figures, Astron. & Ap., in pres
The quest for companions to post-common envelope binaries IV: The 2:1 mean-motion resonance of the planets orbiting NN Serpentis
We present 69 new mid-eclipse times of the young post-common envelope binary
(PCEB) NN Ser, which was previously suggested to possess two circumbinary
planets. We have interpreted the observed eclipse-time variations in terms of
the light-travel time effect caused by two planets, exhaustively covering the
multi-dimensional parameter space by fits in the two binary and ten orbital
parameters. We supplemented the fits by stability calculations for all models
with an acceptable chi-square. An island of secularly stable 2:1 resonant
solutions exists, which coincides with the global chi-square minimum. Our
best-fit stable solution yields current orbital periods P_o = 15.47 yr and P_i
= 7.65 yr and eccentricities e_o = 0.14 and e_i = 0.22 for the outer (o) and
inner (i) planets, respectively. The companions qualify as giant planets, with
masses of 7.0 M_Jup and 1.7 M_Jup for the case of orbits coplanar with that of
the binary. The two-planet model that starts from the present system parameters
has a lifetime greater than 10^8 yr, which significantly exceeds the age of NN
Ser of 10^6 yr as a PCEB. The resonance is characterized by libration of the
resonant variable Theta_1 and circulation of omega_i-omega_o, the difference
between the arguments of periapse of the two planets. No stable non-resonant
solutions were found, and the possibility of a 5:2 resonance suggested
previously by us is now excluded at the 99.3% confidence level.Comment: 8 pages, 8 figure
The Patchy Accretion Disc in HT Cassiopeiae
We have reconstructed the temperatures and surface densities in the quiescent
accretion disc in HT Cas by performing a "Physical Parameter Eclipse Mapping"
analysis of archival UBVR observations. Using a simple hydrogen slab model and
demanding a smooth, maximally artefact-free reconstruction, we derive a formal
distance to HT Cas of 207 +- 10 pc, significantly larger than the 133 +- 14 pc
we derive from a re-analysis of the data in the literature.
We argue that the larger derived distance is probably incorrect but is not
produced by inaccuracies in our spectral model or optimization method. The
discrepancy can be resolved if the emission regions on the disc are patchy with
a filling factor of about 40% of the disc's surface. This solves the problem
with the high effective temperatures in the disc -- reducing them to around 6
500 K within a radius of 0.2 R_L1 -- and reduces the derived temperature of the
white dwarf and/or boundary layer from 22600 to 15500 K.
We discuss several possible sources of the chromospheric emission and its
patchiness, including irradiation of the disc, thermal instabilities,
spiral-wave-like global structures, and magnetically active regions associated
with dynamo action and/or Balbus-Hawley instabilities.Comment: 19 pages, Latex, 25 figures, accepted by MNRA
IVOA Recommendation: Vocabularies in the Virtual Observatory Version 1.19
This document specifies a standard format for vocabularies based on the W3C's
Resource Description Framework (RDF) and Simple Knowledge Organization System
(SKOS). By adopting a standard and simple format, the IVOA will permit
different groups to create and maintain their own specialised vocabularies
while letting the rest of the astronomical community access, use, and combine
them. The use of current, open standards ensures that VO applications will be
able to tap into resources of the growing semantic web. The document provides
several examples of useful astronomical vocabularies
The LIGO HET Response (LIGHETR) Project to Discover and Spectroscopically Follow Optical Transients Associated with Neutron Star Mergers
The LIGO HET Response (LIGHETR) project is an enterprise to follow up optical
transients (OT) discovered as gravitational wave merger sources by the
LIGO/Virgo collaboration (LVC). Early spectroscopy has the potential to
constrain crucial parameters such as the aspect angle. The LIGHETR
collaboration also includes the capacity to model the spectroscopic evolution
of mergers to facilitate a real-time direct comparison of models with our data.
The principal facility is the Hobby-Eberly Telescope. LIGHETR uses the
massively-replicated VIRUS array of spectrographs to search for associated OTs
and obtain early blue spectra and in a complementary role, the low-resolution
LRS-2 spectrograph is used to obtain spectra of viable candidates as well as a
densely-sampled series of spectra of true counterparts. Once an OT is
identified, the anticipated cadence of spectra would match or considerably
exceed anything achieved for GW170817 = AT2017gfo for which there were no
spectra in the first 12 hours and thereafter only roughly once daily. We
describe special HET-specific software written to facilitate the program and
attempts to determine the flux limits to undetected sources. We also describe
our campaign to follow up OT candidates during the third observational campaign
of the LIGO and Virgo Scientific Collaborations. We obtained VIRUS spectroscopy
of candidate galaxy hosts for 5 LVC gravitational wave events and LRS-2 spectra
of one candidate for the OT associated with S190901ap. We identified that
candidate, ZTF19abvionh = AT2019pip, as a possible Wolf-Rayet star in an
otherwise unrecognized nearby dwarf galaxy.Comment: 26 pages, 15 figure
MONET: a MOnitoring NEtwork of Telescopes
In an age of Sm-class telescopes, we should rethink the way we use small telescopes. While 2-4m telescopes are still needed and will continue to be operated largely in the traditional fashion, new lm-class telescopes operated robotically in global networks will enable a wide range of new and exciting scientific and educational projects, both by themselves and in conjunction with much larger telescopes. I describe our plans for such a global network of two lm-class robotic telescopes