86 research outputs found
Superheavy Nuclei in the Relativistic Mean Field Theory
We have carried out a study of superheavy nuclei in the framework of the
Relativistic Mean-Field theory. Relativistic Hartree-Bogoliubov (RHB)
calculations have been performed for nuclei with large proton and neutron
numbers. A finite-range pairing force of Gogny type has been used in the RHB
calculations. The ground-state properties of very heavy nuclei with atomic
numbers Z=100-114 and neutron numbers N=154-190 have been obtained. The results
show that in addition to N=184 the neutron numbers N=160 and N=166 exhibit an
extra stability as compared to their neighbors. For the case of protons the
atomic number Z=106 is shown to demonstrate a closed-shell behavior in the
region of well deformed nuclei about N=160. The proton number Z=114 also
indicates a shell closure. Indications for a doubly magic character at Z=106
and N=160 are observed. Implications of shell closures on a possible synthesis
of superheavy nuclei are discussed.Comment: 29 pages Latex, 13 ps figures, to appear in Nucl. Phys.
Stellar Activity at the End of the Main Sequence: GHRS Observations of the M8 Ve Star VB 10
We present Goddard High Resolution Spectrograph observations of the M8 Ve star VB 10 (equal to G1 752B), located very near the end of the stellar main sequence, and its dM3.5 binary companion G1 752A. These coeval stars provide a test bed for studying whether the outer atmospheres of stars respond to changes in internal structure as stars become fully convective near mass 0.3 solar mass (about spectral type M5), where the nature of the stellar magnetic dynamo presumably changes, and near the transition from red to brown dwarfs near mass 0.08 solar mass (about spectral type M9), when hydrogen burning ceases at the end of the main sequence. We obtain upper limits for the quiescent emission of VB 10 but observe a transition region spectrum during a large flare, which indicates that some type of magnetic dynamo must be present. Two indirect lines of evidence-scaling from the observed X-ray emission and scaling from a time-resolved flare on AD Leo suggest that the fraction of the stellar bolometric luminosity that heats the transition region of VB 10 outside of obvious flares is comparable to, or larger than, that for G1 752A. This suggests an increase in the magnetic heating rates, as measured by L(sub line)/L(sub bol) ratios, across the radiative/convective core boundary and as stars approach the red/brown dwarf boundary. These results provide new constraints for dynamo models and models of coronal and transition-region heating in late-type stars
Quiescent and flare analysis for the chromospherically active star Gl355 (LQHya)
We discuss ROSAT and ASCA observations of the young active star Gl355}.
During the ROSAT observation a strong flare was detected with a peak flux more
than an order of magnitude larger than the quiescent level. Spectral analysis
of the data allows us to study the temperature and emission measure
distribution, and the coronal metal abundance, for the quiescent phase and, in
the case of ROSAT, also during the evolution of the flare. The global coronal
metallicity derived from both ROSAT and ASCA data is
much lower than solar and presumably also much lower than the photospheric
abundance expected for this very young star. The temperature structure of the
quiescent corona was about the same during the various observations, with a
cooler component at MK and a hotter component (to which only ASCA
was sensitive) at MK. During the flare, the low temperature
component remained approximately constant and equal to the quiescent value,
while the high-temperature component was the only one that varied. We have
modeled the flare with the hydrodynamic-decay sustained-heating approach of
Reale at al. (1997) and we have derived a loop semi--length of the order of
stellar radii, i.e. much larger than the dimensions of flares on the
Sun, but comparable with the typical dimensions inferred for other stellar
flares. We have compared the derived loop size with that estimated with a
simpler (but physically inconsistent) approach, finding that for this, as well
for several other stellar flares, the two methods give comparable loop sizes.
Possible causes and consequences of this result are discussed.Comment: A&A, in pres
Rotational modulation of the photospheric and chromospheric activity in the young, single K2-dwarf PW And
High resolution echelle spectra of PW And (HD~1405) have been taken during
eight observing runs from 1999 to 2002. The detailed analysis of the spectra
allow us to determine its spectral type (K2V), mean heliocentric radial
velocity (V_hel = -11.15 km/s) rotational velocity (vsin{i} = 22.6 km/s), and
equivalent width of the lithium line 6707.8 AA (EW(LiI) = 273 mAA). The
kinematic (Galactic Velocity (U, V, W)) confirms its membership of the Local
Association moving group, in agreement with the age (30 to 80 Myrs) inferred
from the color magnitude diagram and the lithium equivalent width. Photospheric
activity (presence of cool spots that disturb the profiles of the photospheric
lines) has been detected as changes in the the bisectors of the cross
correlation function (CCF) resulting of cross-correlate the spectra of PW And
with the spectrum of a non active star of similar spectral type. These
variations of the CCF bisectors are related to the variations in the measured
radial velocities and are modulated with a period similar to the photometric
period of the star. At the same time, chromospheric activity has been analyzed,
using the spectral subtraction technique and simultaneous spectroscopic
observations of the H_alpha, H_beta, NaI D_1 and D_2$, HeI D_3, MgI b triplet,
CaII H&K, and CaII infrared triplet lines. A flare was observed during the last
observing run of 2001, showing an enhancement in the observed chromospheric
lines. A less powerful flare was observed on 2002 August 23. The variations of
the chromospheric activity indicators seem to be related to the photospheric
activity. A correlation between radial velocity, changes in the CCF bisectors
and equivalent width of different chromospheric lines is observed with a
different behaviour between epochs 1999, 2001 and 2002.Comment: Latex file with 20 pages, 21 figures tar'ed gzip'ed. Full postscript
(text, figures and tables) available at
http://www.ucm.es/info/Astrof/users/dmg/pub_dmg.html Accepted for publication
in: Astronomy & Astrophysics (A&A
A Uniform Analysis of 118 Stars with High-Contrast Imaging: Long Period Extrasolar Giant Planets are Rare around Sun-like Stars
We expand on the results of Nielsen et al. (2008), using the null result for
giant extrasolar planets around the 118 target stars from the VLT NACO H and Ks
band planet search (Masciadri et al. 2005), the VLT and MMT Simultaneous
Differential Imaging (SDI) survey (Biller et al. 2007), and the Gemini Deep
Planet Survey (Lafreniere et al. 2007) to set constraints on the population of
giant extrasolar planets. Our analysis is extended to include the planet
luminosity models of Fortney et al. (2008), as well as the correlation between
stellar mass and frequency of giant planets found by Johnson et al. (2007).
Doubling the sample size of FGKM stars strengthens our conclusions: a model for
extrasolar giant planets with power-laws for mass and semi-major axis as giving
by Cumming et al. (2008) cannot, with 95% confidence, have planets beyond 65
AU, compared to the value of 94 AU reported in Nielsen et al. (2008), using the
models of Baraffe et al. (2003). When the Johnson et al. (2007) correction for
stellar mass (which gives fewer Jupiter-mass companions to M stars with respect
to solar-type stars) is applied, however, this limit moves out to 82 AU. For
the relatively new Fortney et al. (2008) models, which predict fainter planets
across most of parameter space, these upper limits, with and without a
correction for stellar mass, are 182 and 234 AU, respectively.Comment: 67 pages, 16 figures, accepted to Ap
American Academy of Periodontology best evidence consensus statement on modifying periodontal phenotype in preparation for orthodontic and restorative treatment
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154649/1/jper10498.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154649/2/jper10498_am.pd
X-ray variability of NGC 2516 stars in the XMM-Newton observations
We present the characteristics of the X-ray variability of stars in the
cluster NGC2516 as derived from XMM-Newton/EPIC/pn data. The X-ray variations
on short (hours), medium (months), and long (years) time scales have been
explored. We detected 303 distinct X-ray sources by analysing six EPIC/pn
observations; 194 of them are members of the cluster. Stars of all spectral
types, from the early-types to the late-M dwarfs, were detected. The
Kolmogorov-Smirnov test applied to the X-ray photon time series shows that, on
short time scales, only a relatively small fraction (ranging from 6% to 31% for
dG and dF, respectively) of the members of NGC2516 are variable with a
confidence level 99%; however, it is possible that the fraction is small
only because of the poor statistics. The time X-ray amplitude distribution
functions (XAD) of a set of dF7-dK2 stars, derived on short (hours) and medium
(months) time scales, seem to suggest that medium-term variations, if present,
have a much smaller amplitude than those on short time scales; a similar result
is also obtained for dK3-dM stars. The amplitude variations of late-type stars
in NGC2516 are consistent with those of the coeval Pleiades stars. Comparing
these data with those of ROSAT/PSPC, collected 7-8 years earlier, and of
ROSAT/HRI, just 4-5 years earlier, we find no evidence of significant
variability on the related time scales, suggesting that long-term variations
due to activity cycles similar to the solar cycle are not common among young
stars. Indications of spectral variability was found in one star whose spectra
at three epochs were available.Comment: 15 pages, 8 figures, published in A&
From Radio to X-ray: Flares on the dMe Flare Star EV Lacertae
We present the results of a campaign to observe flares on the M dwarf flare
star EV Lacertae over the course of two days in 2001 September, utilizing a
combination of radio continuum, optical photometric and spectroscopic,
ultraviolet spectroscopic, and X-ray spectroscopic observations, to
characterize the multi-wavelength nature of flares from this active, single
late-type star. We find flares in every wavelength region in which we observed.
In the multi-wavelength context, the start of the intense radio flare is
coincident with an impulsive optical U-band flare, to within one minute, and
yet there is no signature of an X-ray response. There are other intervals of
time where optical flaring and UV flaring is occurring, but these cannot be
related to the contemporaneous X-ray flaring: the time-integrated luminosities
do not match the instantaneous X-ray flare luminosity, as one would expect for
the Neupert effect. We investigate the probability of chance occurrences of
flares from disparate wavelength regions producing temporal coincidences, but
find that not all the flare associations can be explained by a superposition of
flares due to a high flaring rate. We caution against making causal
associations of multi-wavelength flares based solely on temporal correlations
for high flaring rate stars like EV Lac.Comment: 52 pages, 13 figures, accepted for publication in the Astrophysical
Journa
Recommended from our members
Water-Resources Investigations Report 79-85
From abstract: Channel-routing models were used to route hypothetical releases from reservoirs in the upper Tioga River basin, Pennsylvania. These releases were routed northward down the Tioga River to Lindley, Erwins, and Corning, New York; combined with flows routed down the Cohocton River from Campbell to Corning, New York; and then routed southeastward down the Chemung River from Corning to Chemung, New York. The models used to route the flows of Cohocton and Chemung Rivers accounted for bank-storage discharge and streamflow depletion by well pumpage. In general, 17 years of concurrent streamflow data were available for model calibration and verification
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