51 research outputs found
Nova Geminorum 1912 and the Origin of the Idea of Gravitational Lensing
Einstein's early calculations of gravitational lensing, contained in a
scratch notebook and dated to the spring of 1912, are reexamined. A hitherto
unknown letter by Einstein suggests that he entertained the idea of explaining
the phenomenon of new stars by gravitational lensing in the fall of 1915 much
more seriously than was previously assumed. A reexamination of the relevant
calculations by Einstein shows that, indeed, at least some of them most likely
date from early October 1915. But in support of earlier historical
interpretation of Einstein's notes, it is argued that the appearance of Nova
Geminorum 1912 (DN Gem) in March 1912 may, in fact, provide a relevant context
and motivation for Einstein's lensing calculations on the occasion of his first
meeting with Erwin Freundlich during a visit in Berlin in April 1912. We also
comment on the significance of Einstein's consideration of gravitational
lensing in the fall of 1915 for the reconstruction of Einstein's final steps in
his path towards general relativity.Comment: 31 p
Constraining Disk Parameters of Be Stars using Narrowband H-alpha Interferometry with the NPOI
Interferometric observations of two well-known Be stars, gamma Cas and phi
Per, were collected and analyzed to determine the spatial characteristics of
their circumstellar regions. The observations were obtained using the Navy
Prototype Optical Interferometer equipped with custom-made narrowband filters.
The filters isolate the H-alpha emission line from the nearby continuum
radiation, which results in an increased contrast between the interferometric
signature due to the H-alpha-emitting circumstellar region and the central
star. Because the narrowband filters do not significantly attenuate the
continuum radiation at wavelengths 50 nm or more away from the line, the
interferometric signal in the H-alpha channel is calibrated with respect to the
continuum channels. The observations used in this study represent the highest
spatial resolution measurements of the H-alpha-emitting regions of Be stars
obtained to date. These observations allow us to demonstrate for the first time
that the intensity distribution in the circumstellar region of a Be star cannot
be represented by uniform disk or ring-like structures, whereas a Gaussian
intensity distribution appears to be fully consistent with our observations.Comment: 23 pages, 14 figures, accepted for publication in A
Mass Outflow from Red Giant Stars in M13, M15, and M92
Chromospheric model calculations of the Halpha line for selected red giant
branch (RGB) and asymptotic giant branch (AGB) stars in the globular clusters
M13, M15, and M92 are constructed to derive mass loss rates. The model spectra
are compared to the observations obtained with the Hectochelle on the MMT
telescope. These stars show strong Halpha emissions and blue-shifted Halpha
cores signaling that mass outflow is present in all stars. Outflow velocities
of 3-19 km/s, larger than indicated by Halpha profiles, are needed in the upper
chromosphere to achieve good agreement between the model spectra and the
observations. The resulting mass loss rates range from 0.6*10^{-9} to 5*10^{-9}
Msun/yr, which are about an order of magnitude lower than predicted from
"Reimers' law" or inferred from the infrared excess of similar stars. The mass
loss rate increases slightly with luminosity and with decreasing effective
temperature. Stars in the more metal-rich M13 have higher mass loss rates by a
factor of ~2 than in the metal-poor clusters M15 and M92. A fit to the mass
loss rates is given by: M [Msun/yr] = 0.092 * L^{0.16} * Teff^{-2.02} *
A^{0.37} where A=10^[Fe/H]. Multiple observations of stars revealed one object
in M15, K757, in which the mass outflow increased by a factor of 6 between two
observations separated by 18 months. Other stars showed changes in mass loss
rate by a factor of 1.5 or less.Comment: 28 pages, 10 figures, 3 tables, Accepted in Astronomical Journa
Near-Infrared Variability in the Orion Nebula Cluster
Using the United Kingdom Infrared Telescope on Mauna Kea, we have carried out
a new near-infrared J, H, K monitoring survey of almost a square degree of the
star-forming Orion Nebula Cluster with observations on 120 nights over three
observing seasons, spanning a total of 894 days. We monitored ~15,000 stars
down to J=20 using the WFCAM instrument, and have extracted 1203 significantly
variable stars from our data. By studying variability in young stellar objects
(YSOs) in the H-K, K color-magnitude diagram, we are able to distinguish
between physical mechanisms of variability. Many variables show color behavior
indicating either dust-extinction or disk/accretion activity, but we find that
when monitored for longer periods of time, a number of stars shift between
these two variability mechanisms. Further, we show that the intrinsic timescale
of disk/accretion variability in young stars is longer than that of
dust-extinction variability. We confirm that variability amplitude is
statistically correlated with evolutionary class in all bands and colors. Our
investigations of these 1203 variables have revealed 73 periodic AA Tau type
variables, many large-amplitude and long-period (P > 15 day) YSOs, including
three stars showing widely-spaced periodic brightening events consistent with
circumbinary disk activity, and four new eclipsing binaries. These phenomena
and others indicate the activity of long-term disk/accretion variability
processes taking place in young stars. We have made the light curves and
associated data for these 1203 variables available online.Comment: Corrected typo in author nam
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