922 research outputs found
The Gaia inertial reference frame and the tilting of the Milky Way disk
While the precise relationship between the Milky Way disk and the symmetry
planes of the dark matter halo remains somewhat uncertain, a time-varying disk
orientation with respect to an inertial reference frame seems probable.
Hierarchical structure formation models predict that the dark matter halo is
triaxial and tumbles with a characteristic rate of ~2 rad/Hubble time (~30
muas/yr). These models also predict a time-dependent accretion of gas, such
that the angular momentum vector of the disk should be misaligned with that of
the halo. These effects, as well as tidal effects of the LMC, will result in
the rotation of the angular momentum vector of the disk population with respect
to the quasar reference frame. We assess the accuracy with which the positions
and proper motions from Gaia can be referred to a kinematically non-rotating
system, and show that the spin vector of the transformation from any rigid
self-consistent catalog frame to the quasi-inertial system defined by quasars
should be defined to better than 1 muas/yr. Determination of this inertial
frame by Gaia will reveal any signature of the disk orientation varying with
time, improve models of the potential and dynamics of the Milky Way, test
theories of gravity, and provide new insights into the orbital evolution of the
Sagittarius dwarf galaxy and the Magellanic Clouds.Comment: 16 pages; accepted for publication in Ap
Differential Emission Measure Determination of Collisionally Ionized Plasma: II. Application to Hot Stars
In a previous paper we have described a technique to derive constraints on
the differential emission measure (DEM) distribution, a measure of the
temperature distribution, of collisionally ionized hot plasmas from their X-ray
emission line spectra. We apply this technique to the Chandra/HETG spectra of
all of the nine hot stars available to us at the time this project was
initiated. We find that DEM distributions of six of the seven O stars in our
sample are very similar but that theta Ori has an X-ray spectrum characterized
by higher temperatures. The DEM distributions of both of B stars in our sample
have lower magnitudes than those of the O stars and one, tau Sco, is
characterized by higher temperatures than the other, beta Cru. These results
confirm previous work in which high temperatures have been found for theta Ori
and tau Sco and taken as evidence for channeling of the wind in magnetic
fields, the existence of which are related to the stars' youth. Our results
demonstrate the utility of our method for deriving temperature information for
large samples of X-ray emission line spectra.Comment: The contents of this paper were formerly part of astro-ph/0403603
which was split into two paper
Measurement of Dust Optical Properties in Coalsack
We have used FUSE and Voyager observations of dust scattered starlight in the
neighborhood of the Coalsack Nebula to derive the optical constants of the dust
grains. The albedo is consistent with a value of and the phase
function asymmetry factor with a value of throughout the
spectral range from 900 -- 1200 \AA, in agreement with previous determinations
as well as theoretical predictions. We have now observed two regions (Ophiuchus
and Coalsack) with intense diffuse background radiation and in both cases have
found that the emission is due to light from nearby hot stars scattered by a
relatively thin foreground cloud, with negligible contribution from the
background molecular cloud.Comment: Total 19 pages, Figures 9, Accepted for publication in Astrophysical
Journa
Discovery of an M9.5 Candidate Brown Dwarf in the TW Hydrae Association - DENIS J124514.1-442907
We report the discovery of a fifth candidate substellar system in the ~5-10
Myr TW Hydrae Association - DENIS J124514.1-442907. This object has a NIR
spectrum remarkably similar to that of 2MASS J1139511-315921, a known TW Hydrae
brown dwarf, with low surface gravity features such as a triangular-shaped
H-band, deep H2O absorption, weak alkali lines, and weak hydride bands. We find
an optical spectral type of M9.5 and estimate a mass of <24 M_Jup, assuming an
age of ~5-10 Myr. While the measured proper motion for DENIS J124514.1-442907
is inconclusive as a test for membership, its position in the sky is coincident
with the TW Hydrae Association. A more accurate proper motion measurement,
higher resolution spectroscopy for radial velocity, and a parallax measurement
are needed to derive the true space motion and to confirm its membership.Comment: 8 pages - emulateapj style, 2 figures, 3 tables. Accepted to ApJL.
Fixed typos, added reference, added footnot
Direct determination of quasar redshifts
We present observations of 11 quasars, selected in the range z = 2.2-4.1,
obtained with ESA's Superconducting Tunnel Junction (STJ) camera on the WHT.
Using a single template quasar spectrum, we show that we can determine the
redshifts of these objects to about 1%. A follow-up spectroscopic observation
of one QSO for which our best-fit redshift (z = 2.976) differs significantly
from the tentative literature value (z ~ 2.30) confirms that the latter was
incorrect.Comment: 4 pages, 3 figures; uses aa.cls, psfig.sty, natbib.sty; accepted for
publication in A&A Letter
Spinoza\u27s Cosmopsychism: The Multifaceted Mind of Nature
In my thesis, I will argue that while at first glance Spinoza\u27s system seems to conflict with itself and provide an explanation of the mind that can be more confusing than helpful, its aspect of panpsychism provides a sort of lifeline for understanding. Not only does Spinoza\u27s cosmopsychism make his system cohesive, but it can also be independently motivated by ideas that are compatible with the more contemporary intuitions in philosophy of mind. I will explore the specific nature of this universal mentality, how Spinoza\u27s particular genre of panpsychism relates to other theories of panpsychism, and how his conception of omnipresent mind affects our understanding of consciousness and its place in the world
Five New Transits of the Super-Neptune HD 149026
We present new photometry of HD 149026 spanning five transits of its
"super-Neptune" planet. In combination with previous data, we improve upon the
determination of the planet-to-star radius ratio: R_p/R_star =
0.0491^{+0.0018}_{-0.0005}. We find the planetary radius to be 0.71 +/- 0.05
R_Jup, in accordance with previous theoretical models invoking a high metal
abundance for the planet. The limiting error is the uncertainty in the stellar
radius. Although we find agreement among four different ways of estimating the
stellar radius, the uncertainty remains at 7%. We also present a refined
transit ephemeris and a constraint on the orbital eccentricity and argument of
pericenter, e cos(omega) = -0.0014 +/- 0.0012, based on the measured interval
between primary and secondary transits.Comment: To appear in ApJ [19 pages
Coronal Emission Measures and Abundances for Moderately Active K Dwarfs Observed by Chandra
We have used Chandra to resolve the nearby 70 Oph (K0 V+K5 V) and 36 Oph (K1
V+K1 V) binary systems for the first time in X-rays. The LETG/HRC-S spectra of
all four of these stars are presented and compared with an archival LETG
spectrum of another moderately active K dwarf, Epsilon Eri. Coronal densities
are estimated from O VII line ratios and emission measure distributions are
computed for all five of these stars. We see no substantial differences in
coronal density or temperature among these stars, which is not surprising
considering that they are all early K dwarfs with similar activity levels.
However, we do see significant differences in coronal abundance patterns.
Coronal abundance anomalies are generally associated with the first ionization
potential (FIP) of the elements. On the Sun, low-FIP elements are enhanced in
the corona relative to high-FIP elements, the so-called "FIP effect." Different
levels of FIP effect are seen for our stellar sample, ranging from 70 Oph A,
which shows a prominent solar-like FIP effect, to 70 Oph B, which has no FIP
bias at all or possibly even a weak inverse FIP effect. The strong abundance
difference exhibited by the two 70 Oph stars is unexpected considering how
similar these stars are in all other respects (spectral type, age, rotation
period, X-ray flux). It will be difficult for any theoretical explanation for
the FIP effect to explain how two stars so similar in all other respects can
have coronae with different degrees of FIP bias. Finally, for the stars in our
sample exhibiting a FIP effect, a curious difference from the solar version of
the phenomenon is that the data seem to be more consistent with the high-FIP
elements being depleted in the corona rather than a with a low-FIP enhancementComment: 35 pages, 8 figures, AASTEX v5.0 plus EPSF extensions in mkfig.sty;
accepted by Ap
Chromospheric CaII Emission in Nearby F, G, K, and M stars
We present chromospheric CaII activity measurements, rotation periods and
ages for ~1200 F-, G-, K-, and M- type main-sequence stars from ~18,000
archival spectra taken at Keck and Lick Observatories as a part of the
California and Carnegie Planet Search Project. We have calibrated our
chromospheric S values against the Mount Wilson chromospheric activity data.
From these measurements we have calculated median activity levels and derived
R'HK, stellar ages, and rotation periods for 1228 stars, ~1000 of which have no
previously published S values. We also present precise time series of activity
measurements for these stars.Comment: 62 pages, 7 figures, 1 table. Second (extremely long) table is
available at http://astro.berkeley.edu/~jtwright/CaIIdata/tab1.tex Accepted
by ApJ
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