331 research outputs found
The White Dwarfs within 25 Parsecs of the Sun: Kinematics and Spectroscopic Subtypes
We present the fractional distribution of spectroscopic subtypes, range and
distribution of surface temperatures, and kinematical properties of the white
dwarfs within 25pc of the sun. There is no convincing evidence of halo white
dwarfs in the total 25 pc sample of 224 white dwarfs. There is also little to
suggest the presence of genuine thick disk subcomponent members within 25
parsecs. It appears that the entire 25 pc sample likely belong to the thin
disk. We also find no significant kinematic differences with respect to
spectroscopic subtypes. The total DA to non-DA ratio of the 25 pc sample is
1.8, a manifestation of deepening envelope convection which transforms DA stars
with sufficiently thin H surface layers into non-DAs. We compare this ratio
with the results of other studies. We find that at least 11% of the white
dwarfs within 25 parsecs of the sun (the DAZ and DZ stars) have photospheric
metals that likely originate from accretion of circumstellar material (debris
disks) around them. If this interpretation is correct, then it suggests the
possibility that a similar percentage have planets, asteroid-like bodies or
debris disks orbiting them. Our volume-limited sample reveals a pileup of DC
white dwarfs at the well-known cutoff in DQ white dwarfs at Tef about 6000K.
Mindful of small number statistics, we speculate on its possible evolutionary
significance. We find that the incidence of magnetic white dwarfs in the 25 pc
sample is at least 8%, in our volume-limited sample, dominated by cool white
dwarfs. We derive approximate formation rates of DB and DQ degenerates and
present a preliminary test of the evolutionary scenario that all cooling DB
stars become DQ white dwarfs via helium convective dredge-up with the diffusion
tail of carbon extending upward from their cores.Comment: Accepted for publication in The Astronomical Journa
Testing the Stellar Rotation vs. Age Paradigm Using Wide Binaries in the Kepler & K2 Fields
Essential to understanding the history of the Galaxyâs stellar populations, ages are among the most difficult to measure properties of stars. Accurate stellar ages would provide key leverage on problems ranging from the habitability of exoplanets to the Galaxyâs chemical evolution and age. Gyrochronology, the empirical relation between rotation and age, is believed to be among the best current methods of stellar age determination. Using several hundred wide non-interacting binaries identified in the Kepler mission âK2â extended mission fields we are testing this paradigm. Such wide pairs afford a unique opportunity to unravel and calibrate the effects of mass, rotation, activity and age because components of a given binary should have rotation and activity levels consistent with their masses and a common age. We present preliminary results for several dozen wide pairs in which both components exhibit detectable rotational modulation
The 25 Parsec Local White Dwarf Population
We have extended our detailed survey of the local white dwarf population from 20 pc to 25 pc, effectively doubling the sample volume, which now includes 232 stars. In the process new stars within 20 pc have been added, a more uniform set of distance estimates as well as improved spectral and binary classifications are available. The present 25 pc sample is estimated to be about 68% complete (the corresponding 20 pc sample is now 86% complete). The space density of white dwarfs is unchanged at 4.8 ± 0.5 x 10-3 pc-3. This new study includes a white dwarf mass distribution and luminosity function based on the 232 stars in the 25 pc sample. We find a significant excess of single stars over systems containing one or more companions (74% vs 26%). This suggests mechanisms that result in the loss of companions during binary system evolution. In addition this updated sample exhibits a pronounced deficiency of nearby âSirius-Likeâ systems. Eleven such systems were found within the 20 pc volume vs, only one additional system found in the volume between 20 pc and 25 pc. An estimate of white dwarf birth rates during the last ~ 8 Gyr is derived from individual remnant cooling ages. A discussion of likely ways new members of the local sample may be found is provided
Shifts in dominant tree mycorrhizal associations in response to anthropogenic impacts
Plant-fungal symbioses play critical roles in vegetation dynamics and nutrient cycling, modulating the impacts of global changes on ecosystem functioning. Here, we used forest inventory data consisting of more than 3 million trees to develop a spatially resolved âmycorrhizal tree mapâ of the contiguous United States. We show that abundances of the two dominant mycorrhizal tree groupsâarbuscular mycorrhizal (AM) and ectomycorrhizal treesâare associated primarily with climate. Further, we show that anthropogenic influences, primarily nitrogen (N) deposition and fire suppression, in concert with climate change, have increased AM tree dominance during the past three decades in the eastern United States. Given that most AM-dominated forests in this region are underlain by soils with high N availability, our results suggest that the increasing abundance of AM trees has the potential to induce nutrient acceleration, with critical consequences for forest productivity, ecosystem carbon and nutrient retention, and feedbacks to climate change
The Chromospheric Activity and Ages of M Dwarf Stars in Wide Binary Systems
We investigate the relationship between age and chromospheric activity for
139 M dwarf stars in wide binary systems with white dwarf companions. The age
of each system is determined from the cooling age of its white dwarf component.
The current limit for activity-age relations found for M dwarfs in open
clusters is 4 Gyr. Our unique approach to finding ages for M stars allows for
the exploration of this relationship at ages older than 4 Gyr. The general
trend of stars remaining active for a longer time at later spectral type is
confirmed. However, our larger sample and greater age range reveals additional
complexity in assigning age based on activity alone. We find that M dwarfs in
wide binaries older than 4 Gyr depart from the log-linear relation for clusters
and are found to have activity at magnitudes, colors and masses which are
brighter, bluer and more massive than predicted by the cluster relation. In
addition to our activity-age results, we present the measured radial velocities
and complete space motions for 161 white dwarf stars in wide binaries.Comment: 22 pages including 9 figures and 5 tables. Accepted for publication
in The Astronomical Journa
The galactic population of white dwarfs
Original paper can be found at: http://www.iop.org/EJ/conf DOI: 10.1088/1742-6596/172/1/012004 [16th European White Dwarfs Workshop]The contribution of white dwarfs of the different Galactic populations to the stellar content of our Galaxy is only poorly known. Some authors claim a vast population of halo white dwarfs, which would be in accordance with some investigations of the early phases of Galaxy formation claiming a top-heavy initialâ massâ function. Here, I present a model of the population of white dwarfs in the Milky Way based on observations of the local white dwarf sample and a standard model of Galactic structure. This model will be used to estimate the space densities of thin disc, thick disc and halo white dwarfs and their contribution to the baryonic mass budget of the Milky Way. One result of this investigation is that white dwarfs of the halo population contribute a large fraction of the Galactic white dwarf number count, but they are not responsible for the lion's share of stellar mass in the Milky Way. Another important result is the substantial contribution of the â often neglected â population of thick disc white dwarfs. Misclassification of thick disc white dwarfs is responsible for overestimates of the halo population in previous investigations.Peer reviewe
Steps Toward Determination of the Size and Structure of the Broad-Line Region in Active Galactic Nuclei. XII. Ground-Based Monitoring of 3C 390.3
Results of a ground-based optical monitoring campaign on 3C 390.3 in 1994-1995 are presented. The broadband fluxes (B, V, R, and I), the spectrophotometric optical continuum flux and the Fλ(5177 Ă
),integrated emission-line fluxes of Há”Ș HÎČ, Hᔧ, He I λ5876 and He II λ4686 all show a nearly monotonic increase with episodes of milder short-term variations superposed. The amplitude of the continuum variations increases with decreasing wavelength (4400-9000 Ă
). The optical continuum variations follow the variations in the ultraviolet and X-ray with time delays, measured from the centroids of the cross-correlation functions, typically around 5 days, but with uncertainties also typically around 5 days; zero time delay between the high-energy and low-energy continuum variations cannot be ruled out. The strong optical emission lines Há”Ș HÎČ, Hᔧ, He I λ5876 respond to the high-energy continuum variations with time delays typically about 20 days, with uncertainties of about 8 days. There is some evidence that He II λ4686 responds somewhat more rapidly, with a time delay of around 10 days, but again, the uncertainties are quite large (~8 days). The mean and rms spectra of the Há”Ș and HÎČ line profiles provide indications for the existence of at least three distinct components located at ±4000 and 0 km s-1 relative to the line peak. The emission-line profle variations are largest near line center
A New Look at the Local White Dwarf Population
We have conducted a detailed new survey of the local population of white dwarfs lying within 20 pc of the Sun. A new revised catalog of local white dwarfs containing 122 entries (126 individual degenerate stars) is presented. This list contains 27 white dwarfs not included in a previous list from 2002, as well as new and recently published trigonometric parallaxes. In several cases new members of the local white dwarf population have come to light through accurate photometric distance estimates. In addition, a suspected new double degenerate system (WD 0423+120) has been identified. The 20 pc sample is currently estimated to be 80% complete. Using a variety of recent spectroscopic, photometric, and trigonometric distance determinations, we re-compute a space density of 4.8 ± 0.5 Ă 10â3 pcâ3 corresponding to a mass density of 3.2 ± 0.3 Ă 10â3 M pcâ3 from the complete portion of the sample within 13 pc. We find an overall mean mass for the local white dwarfs of 0.665 M, a value larger than most other non-volume-limited estimates. Although the sample is small, we find no evidence of a correlation between mass and temperature in which white dwarfs below 13,000 K are systematically more massive than those above this temperature. Within 20 pc 25% of the white dwarfs are in binary systems (including double degenerate systems). Approximately 6% are double degenerates and 6.5% are Sirius-like systems. The fraction of magnetic white dwarfs in the local population is found to be 13%
The White Dwarfs within 20 Parsecs of the Sun: Kinematics and Statistics
We present the kinematical properties, distribution of spectroscopic
subtypes, stellar population subcomponents of the white dwarfs within 20 pc of
the sun. We find no convincing evidence of halo white dwarfs in the total 20 pc
sample of 129 white dwarfs nor is there convincing evidence of genuine thick
disk subcomponent members within 20 parsecs. Virtually the entire 20 pc sample
likely belongs to the thin disk. The total DA to non-DA ratio of the 20 pc
sample is 1.6, a manifestation of deepening envelope convection which
transforms DA stars with sufficiently thin H surface layers into non-DAs. The
addition of 5 new stars to the 20 pc sample yields a revised local space
density of white dwarfs of M_{\sun}/yr and a
corresponding mass density of M_{\sun}/pc.
We find that at least 15% of the white dwarfs within 20 parsecs of the sun (the
DAZ and DZ stars) have photospheric metals that possibly originate from
accretion of circumstellar material (debris disks) around them. If this
interpretation is correct, this suggests the possibility that the same
percentage have planets or asteroid-like bodies orbiting them.Comment: Accepted for publication in The Astronomical Journa
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