4,522 research outputs found
Bayesian analysis of resolved stellar spectra: application to MMT/Hectochelle Observations of the Draco dwarf spheroidal
We introduce a Bayesian method for fitting faint, resolved stellar spectra in
order to obtain simultaneous estimates of redshift and stellar-atmospheric
parameters. We apply the method to thousands of spectra---covering 5160-5280
Angs. at resolution R~20,000---that we have acquired with the MMT/Hectochelle
fibre spectrograph for red-giant and horizontal branch candidates along the
line of sight to the Milky Way's dwarf spheroidal satellite in Draco. The
observed stars subtend an area of ~4 deg^2, extending ~3 times beyond Draco's
nominal `tidal' radius. For each spectrum we tabulate the first four
moments---central value, variance, skewness and kurtosis---of posterior
probability distribution functions representing estimates of the following
physical parameters: line-of-sight velocity v_los, effective temperature
(T_eff), surface gravity (logg) and metallicity ([Fe/H]). After rejecting
low-quality measurements, we retain a new sample consisting of 2813 independent
observations of 1565 unique stars, including 1879 observations for 631 stars
with (as many as 13) repeat observations. Parameter estimates have median
random errors of sigma_{v_los}=0.88 km/s, sigma_{T_eff}=162 K, sigma_logg=0.37
dex and sigma_[Fe/H]=0.20 dex. Our estimates of physical parameters distinguish
~470 likely Draco members from interlopers in the Galactic foreground.Comment: published in Monthly Notices of the Royal Astronomical Society, all
data are publicly available at the following address:
http://www.andrew.cmu.edu/user/mgwalker/hectochelle
Clean Kinematic Samples in Dwarf Spheroidals: An Algorithm for Evaluating Membership and Estimating Distribution Parameters When Contamination is Present
(abridged) We develop an algorithm for estimating parameters of a
distribution sampled with contamination, employing a statistical technique
known as ``expectation maximization'' (EM). Given models for both member and
contaminant populations, the EM algorithm iteratively evaluates the membership
probability of each discrete data point, then uses those probabilities to
update parameter estimates for member and contaminant distributions. The EM
approach has wide applicability to the analysis of astronomical data. Here we
tailor an EM algorithm to operate on spectroscopic samples obtained with the
Michigan-MIKE Fiber System (MMFS) as part of our Magellan survey of stellar
radial velocities in nearby dwarf spheroidal (dSph) galaxies. These samples are
presented in a companion paper and contain discrete measurements of
line-of-sight velocity, projected position, and Mg index for ~1000 - 2500 stars
per dSph, including some fraction of contamination by foreground Milky Way
stars. The EM algorithm quantifies both dSph and contaminant distributions,
returning maximum-likelihood estimates of the means and variances, as well as
the probability that each star is a dSph member. Applied to our MMFS data, the
EM algorithm identifies more than 5000 probable dSph members. We test the
performance of the EM algorithm on simulated data sets that represent a range
of sample size, level of contamination, and amount of overlap between dSph and
contaminant velocity distributions. The simulations establish that for samples
ranging from large (N ~3000) to small (N~30), the EM algorithm distinguishes
members from contaminants and returns accurate parameter estimates much more
reliably than conventional methods of contaminant removal (e.g., sigma
clipping).Comment: Accepted for publication in The Astronomical Journal. Download pdf
with full-resolution figures from
http://www.ast.cam.ac.uk/~walker/dsph_em.pd
Spatial and temporal variation in otolith chemistry for tautog (Tautoga onitis) in Narragansett Bay and Rhode Island coastal ponds
The elemental composition of otoliths may provide valuable
information for establishing connectivity between fish nursery grounds and adult fish populations. Concentrations
of Rb, Mg, Ca, Mn, Sr, Na, K, Sr, Pb, and Ba were determined
by using solution-based inductively coupled plasma mass spectrometry in otoliths of young-of-the year tautog
(Tautoga onitis) captured in nursery areas along the Rhode Island coast during two consecutive years. Stable oxygen (δ18O) and carbon (δ13C) isotopic ratios in young-of-the year otoliths were also analyzed with isotope ratio mass spectrometry. Chemical signatures differed significantly
among the distinct nurseries within Narragansett Bay and the coastal ponds across years. Significant differences
were also observed within nurseries from year to year. Classification accuracy to each of the five tautog nursery areas ranged from 85% to 92% across years. Because accurate
classification of juvenile tautog nursery sites was achieved, otolith chemistry can potentially be used as
a natural habitat tag
On Kinematic Substructure in the Sextans Dwarf Spheroidal Galaxy
We present multifiber echelle radial velocity results for 551 stars in the
Sextans dwarf spheroidal galaxy and identify 294 stars as probable Sextans
members. The projected velocity dispersion profile of the binned data remains
flat to a maximum angular radius of . We introduce a nonparametric
technique for estimating the projected velocity dispersion surface, and use
this to search for kinematic substructure. Our data do not confirm previous
reports of a kinematically distinct stellar population at the Sextans center.
Instead we detect a region near the Sextans core radius that is kinematically
colder than the overall Sextans sample with 95% confidence.Comment: accepted for publication in ApJ Letters; 4 figures (2 color
The mossy north: An inverse latitudinal diversity gradient in European bryophytes
Rubén G. Mateo [et al.]It remains hotly debated whether latitudinal diversity gradients are common across taxonomic groups and whether a single mechanism can explain such gradients. Investigating species richness (SR) patterns of European land plants, we determine whether SR increases with decreasing latitude, as predicted by theory, and whether the assembly mechanisms differ among taxonomic groups. SR increases towards the south in spermatophytes, but towards the north in ferns and bryophytes. SR patterns in spermatophytes are consistent with their patterns of beta diversity, with high levels of nestedness and turnover in the north and in the south, respectively, indicating species exclusion towards the north and increased opportunities for speciation in the south. Liverworts exhibit the highest levels of nestedness, suggesting that they represent the most sensitive group to the impact of past climate change. Nevertheless, although the extent of liverwort species turnover in the south is substantially and significantly lower than in spermatophytes, liverworts share with the latter a higher nestedness in the north and a higher turn-over in the south, in contrast to mosses and ferns. The extent to which the similarity in the patterns displayed by spermatophytes and liverworts reflects a similar assembly mechanism remains, however, to be demonstrated.Peer reviewe
Resolving the timing problem of the globular clusters orbiting the Fornax dwarf galaxy
We re-investigate the old problem of the survival of the five globular
clusters orbiting the Fornax dwarf galaxy in both standard and modified
Newtonian dynamics. For the first time in the history of the topic, we use
accurate mass models for the Fornax dwarf, obtained through Jeans modelling of
the recently published line of sight velocity dispersion data, and we are also
not resigned to circular orbits for the globular clusters. Previously conceived
problems stem from fixing the starting distances of the globulars to be less
than half the tidal radius. We relax this constraint since there is absolutely
no evidence for it and show that the dark matter paradigm, with either cusped
or cored dark matter profiles, has no trouble sustaining the orbits of the two
least massive globular clusters for a Hubble time almost regardless of their
initial distance from Fornax. The three most massive globulars can remain in
orbit as long as their starting distances are marginally outside the tidal
radius. The outlook for modified Newtonian dynamics is also not nearly as bleak
as previously reported. Although dynamical friction inside the tidal radius is
far stronger in MOND, outside dynamical friction is negligible due to the
absence of stars. This allows highly radial orbits to survive, but more
importantly circular orbits at distances more than 85% of Fornax's tidal radius
to survive indefinitely. The probability of the globular clusters being on
circular orbits at this distance compared with their current projected
distances is discussed and shown to be plausible. Finally, if we ignore the
presence of the most massive globular (giving it a large line of sight
distance) we demonstrate that the remaining four globulars can survive within
the tidal radius for the Hubble time with perfectly sensible orbits.Comment: 8 pages, 10 figures, 1 table, MNRAS in pres
Plasticity of Alarm-call Response Development in Belding’s Ground Squirrels ( Spermophilus beldingi , Sciuridae)
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72444/1/j.1439-0310.1999.00389.x.pd
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