113 research outputs found
Monitoring solar-type stars for luminosity variations
Since 1984, researchers have made more than 1500 differential photometric b (471 nm) and y (551 nm) measurements of three dozen solar-like lower main sequence stars whose chromospheric activity was previosly studied by O. C. Wilson. Here, researchers describe their methodology and the statistical tests used to distinguish intrinsic stellar variability from observational and instrument errors. The incidence of detected variability among the program and comparison stars is summarized. Among the 100 plus pairs of stars measured differentially, only a dozen were found that were unusually constant, with peak-to-peak amplitudes of seasonal mean brightness smaller than 0.3 percent (0.003 mag) over a two-to-three-year interval
Results from the 2014 november 15th. multi-chord stellar occultation by the TNO (229762) 2007 UK126
Benedetti-Rossi, G. et. al.We present results derived from the first multi-chord stellar occultation by the trans-Neptunian object (229762) 2007 UK, observed on 2014 November 15. The event was observed by the Research and Education Collaborative Occultation Network project and International Occultation Timing Association collaborators throughout the United States. Use of two different data analysis methods obtain a satisfactory fit to seven chords, yielding an elliptical fit to the chords with an equatorial radius of R = 338 km and equivalent radius of R = 319 km. A circular fit also gives a radius of R = 324 km. Assuming that the object is a Maclaurin spheroid with indeterminate aspect angle, and using two published absolute magnitudes for the body, we derive possible ranges for geometric albedo between p = 0.159 and p = 0.189 , and for the body oblateness between ∈ = 0.105 and ∈ = 0.118 . For a nominal rotational period of 11.05 hr, an upper limit for density of ρ = 1740 kg m is estimated for the body.The RECON project would not be possible without all of the support from our community teams (teachers, students, and community members) and was funded by NSF grants AST-1212159, AST-1413287, and AST-1413072. Special thanks to Dean and Starizona for their support of RECON above and beyond the usual bounds of a commercial relationship.
Part of the research leading to these results has received funding from the European Research Council under the European Community's H2020 (2014-2020/ERC Grant Agreement n 669416 >LUCKY STAR>).
Funding from Spanish grant AYA-2014-56637-C2-1-P is acknowledged, as is the Proyecto de Excelencia de la Junta de Andaluca, J. A. 2012-FQM1776. R.D. acknowledges the support of MINECO for his Ramon y Cajal Contract. FEDER funds are also acknowledged.
A.D.O. is thankful for the support of the CAPES (BEX 9110/12-7) FAPERJ/PAPDRJ (E-26/200.464/2015) grants.
G.B.R. is thankful for the support of CAPES/Brazil and FAPERJ (Grant E-01/2015/209545).
J.I.B.C. acknowledges CNPq for a PQ2 fellowship (process number 308489/2013-6).
L.G. thanks the support from CONACYT through grant 167236.
M.A. thanks the CNPq (Grants 473002/2013-2 and 308721/2011-0) and FAPERJ (Grant E-26/111.488/2013).
P.S.-S. acknowledges that part of the research leading to these results has received funding from the European Unions Horizon 2020 Research and Innovation Programme, under Grant Agreement no 687378.
R.V.M. acknowledges the following grants: CNPq-306885/2013, CAPES/Cofecub-2506/2015, FAPERJ/PAPDRJ-45/2013, FAPERJ/CNE/05-2015.Peer Reviewe
An Untriggered Search for Optical Bursts
We present an untriggered search for optical bursts with the ROTSE-I
telephoto array. Observations were taken which monitor an effective 256 square
degree field continuously over 125 hours to m_{ROTSE}=15.7. The uniquely large
field, moderate limiting magnitude and fast cadence of 10 minutes permits
transient searches in a new region of sensitivity. Our search reveals no
candidate events. To quantify this result, we simulate potential optical bursts
with peak magnitude, m_{p}, at t=10 s, which fade as f=(\frac{t}{t_{0}})
^{\alpha_{t}}, where \alpha_t < 0. Simple estimates based on observational
evidence indicate that a search of this sensitivity begins to probe the
possible region occupied by GRB orphan afterglows. Our observing protocol and
image sensitivity result in a broad region of high detection efficiency for
light curves to the bright and slowly varying side of a boundary running from
[\alpha_{t},m_{p}]=[-2.0,6.0] to [-0.3,13.2]. Within this region, the
integrated rate of brief optical bursts is less than 1.1\times 10^{-8} {\rm
s}^{-1} {\rm deg}^{-2}. At 22 times the observed GRB rate from BATSE,
this suggests a limit on \frac{\theta_{opt}}{\theta_{\gamma}}\lesssim 5 where
\theta_{opt} and \theta_{\gamma} are the optical and gamma-ray collimation
angles, respectively. Several effects might explain the absence of optical
bursts, and a search of the kind described here but more sensitive by about 4
magnitudes should offer a more definitive probe.Comment: 8 pages, 6 figures, 1 tabl
HV 11423: The Coolest Supergiant in the SMC
We call attention to the fact that one of the brightest red supergiants in
the SMC has recently changed its spectral type from K0-1 I (December 2004) to
M4 I (December 2005) and back to K0-1 I (September 2006). An archival spectrum
from the Very Large Telescope reveals that the star was even cooler (M4.5-M5 I)
in December 2001. By contrast, the star was observed to be an M0 I in both
October 1978 and October 1979. The M4-5 I spectral types is by far the latest
type seen for an SMC supergiant, and its temperature in that state places it
well beyond the Hayashi limit into a region of the H-R diagram where the star
should not be in hydrostatic equilibrium. The star is variable by nearly 2 mag
in V, but essentially constant in K. Our modeling of its spectral energy
distribution shows that the visual extinction has varied during this time, but
that the star has remained essentially constant in bolometric luminosity. We
suggest that the star is currently undergoing a period of intense instability,
with its effective temperature changing from 4300 K to 3300 K on the time-scale
of months. It has one of the highest 12-micron fluxes of any RSG in the SMC,
and we suggest that the variability at V is due primarily to changes in
effective temperature, and secondly, due to changes in the local extinction due
to creation and dissipation of circumstellar dust. We speculate that the star
may be nearing the end of its life.Comment: Accepted by the Astrophysical Journa
A Candidate Young Massive Planet in Orbit around the Classical T Tauri Star CI Tau
The ~2 Myr old classical T Tauri star CI Tau shows periodic variability in
its radial velocity (RV) variations measured at infrared (IR) and optical
wavelengths. We find that these observations are consistent with a massive
planet in a ~9-day period orbit. These results are based on 71 IR RV
measurements of this system obtained over 5 years, and on 26 optical RV
measurements obtained over 9 years. CI Tau was also observed photometrically in
the optical on 34 nights over ~one month in 2012. The optical RV data alone are
inadequate to identify an orbital period, likely the result of star spot and
activity induced noise for this relatively small dataset. The infrared RV
measurements reveal significant periodicity at ~9 days. In addition, the full
set of optical and IR RV measurements taken together phase coherently and with
equal amplitudes to the ~9 day period. Periodic radial velocity signals can in
principle be produced by cool spots, hot spots, and reflection of the stellar
spectrum off the inner disk, in addition to resulting from a planetary
companion. We have considered each of these and find the planet hypothesis most
consistent with the data. The radial velocity amplitude yields an Msin(i) of
~8.1 M_Jup; in conjunction with a 1.3 mm continuum emission measurement of the
circumstellar disk inclination from the literature, we find a planet mass of
~11.3 M_Jup, assuming alignment of the planetary orbit with the disk.Comment: 61 pages, 13 figures, accepted for publication in The Astrophysical
Journa
The radial pulsation of AI Aurigae
We present an analysis of eleven years of Stromgren by photometry of the red
semiregular variable star AI Aurigae. An early period determination of 63.9
days is confirmed by the long-term light curve behaviour. The light curve shows
semi-regular changes with a mean period of 65 days reaching an amplitude of 0.6
mag in some cycles. The b-y colour changes perfectly parallel the V light
curve, suggesting radial oscillation to be the main reason for the observed
variations. We estimate the main characteristics of the star (mass, radius,
effective temperature) that suggest radial pulsation in fundamental or first
overtone mode.Comment: 7 pages, 3 figures, accepted for publication in A&
Patterns of photometric and chromospheric variation among Sun-like stars: A 20-year perspective
We examine patterns of variation of 32 primarily main sequence stars,
extending our previous 7-12 year time series to 13-20 years by combining b, y
data from Lowell Observatory with similar data from Fairborn Observatory.
Parallel chromospheric Ca II H and K emission data from the Mount Wilson
Observatory span the entire interval. The extended data strengthen the
relationship between chromospheric and photometric variation derived
previously. Twenty-seven stars are deemed variable. On a year-to-year timescale
young active stars become fainter when their Ca II emission increases while
older less active stars such as the Sun become brighter when their Ca II
emission increases. The Sun's total irradiance variation, scaled to the b and y
filter photometry, still appears to be somewhat smaller than stars in our
limited sample with similar mean chromospheric activity, but we now regard this
discrepancy as probably due mainly to our limited stellar sampl
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