454 research outputs found
A Keck/HIRES Doppler Search for Planets Orbiting Metal-Poor Dwarfs. I. Testing Giant Planet Formation and Migration Scenarios
We describe a high-precision Doppler search for giant planets orbiting a
well-defined sample of metal-poor dwarfs in the field. This experiment
constitutes a fundamental test of theoretical predictions which will help
discriminate between proposed giant planet formation and migration models. We
present here details on the survey as well as an overall assessment of the
quality of our measurements, making use of the results for the stars that show
no significant velocity variation.Comment: 25 pages, 7 figures, accepted for publication in the Astrophysical
Journa
Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 parsecs: The Northern Sample I
We have embarked on a project, under the aegis of the Nearby Stars (NStars)/
Space Interferometry Mission Preparatory Science Program to obtain spectra,
spectral types, and, where feasible, basic physical parameters for the 3600
dwarf and giant stars earlier than M0 within 40 parsecs of the sun. In this
paper we report on the results of this project for the first 664 stars in the
northern hemisphere. These results include precise, homogeneous spectral types,
basic physical parameters (including the effective temperature, surface gravity
and the overall metallicity, [M/H]) and measures of the chromospheric activity
of our program stars. Observed and derived data presented in this paper are
also available on the project's website at http://stellar.phys.appstate.edu/
Behavior of Li abundances in solar-analog stars II. Evidence of the connection with rotation and stellar activity
We previously attempted to ascertain why the Li I 6708 line-strengths of
Sun-like stars differ so significantly despite the superficial similarities of
stellar parameters. We carried out a comprehensive analysis of 118 solar
analogs and reported that a close connection exists between the Li abundance
A_Li and the line-broadening width (v_r+m; mainly contributed by rotational
effect), which led us to conclude that stellar rotation may be the primary
control of the surface Li content. To examine our claim in more detail, we
study whether the degree of stellar activity exhibits a similar correlation
with the Li abundance, which is expected because of the widely believed close
connection between rotation and activity. We measured the residual flux at the
line center of the strong Ca II 8542 line, r_0(8542), known to be a useful
index of stellar activity, for all sample stars using newly acquired spectra in
this near-IR region. The projected rotational velocity (v_e sin i) was
estimated by subtracting the macroturbulence contribution from v_r+m that we
had already established. A remarkable (positive) correlation was found in the
A_Li versus (vs.) r_0(8542) diagram as well as in both the r_0(8542) vs. v_e
sin i and A_Li vs. v_e sin i diagrams, as had been expected. With the
confirmation of rotation-dependent stellar activity, this clearly shows that
the surface Li abundances of these solar analogs progressively decrease as the
rotation rate decreases. Given this observational evidence, we conclude that
the depletion of surface Li in solar-type stars, probably caused by effective
envelope mixing, operates more efficiently as stellar rotation decelerates. It
may be promising to attribute the low-Li tendency of planet-host G dwarfs to
their different nature in the stellar angular momentum.Comment: 12 pages, 9 figures; accepted for publication in Astron. Astrophys
The stellar atmosphere simulation code Bifrost
Context: Numerical simulations of stellar convection and photospheres have
been developed to the point where detailed shapes of observed spectral lines
can be explained. Stellar atmospheres are very complex, and very different
physical regimes are present in the convection zone, photosphere, chromosphere,
transition region and corona. To understand the details of the atmosphere it is
necessary to simulate the whole atmosphere since the different layers interact
strongly. These physical regimes are very diverse and it takes a highly
efficient massively parallel numerical code to solve the associated equations.
Aims: The design, implementation and validation of the massively parallel
numerical code Bifrost for simulating stellar atmospheres from the convection
zone to the corona.
Methods: The code is subjected to a number of validation tests, among them
the Sod shock tube test, the Orzag-Tang colliding shock test, boundary
condition tests and tests of how the code treats magnetic field advection,
chromospheric radiation, radiative transfer in an isothermal scattering
atmosphere, hydrogen ionization and thermal conduction.
Results: Bifrost completes the tests with good results and shows near linear
efficiency scaling to thousands of computing cores
Differential rotation measurement of soft X-Ray corona
The aim of this paper is to study the latitudinal variation in the solar
rotation in soft X-ray corona. The time series bins are formed on different
latitude regions of the solar full disk (SFD) images that extend from 80 degree
South to 80 degree North. These SFD images are obtained with the soft X-ray
telescope (SXT) on board the Yohkoh solar observatory. The autocorrelation
analyses are performed with the time series that track the SXR flux modulations
in the solar corona. Then for each year, extending from 1992 to 2001, we obtain
the coronal sidereal rotation rate as a function of the latitude. The present
analysis from SXR radiation reveals that; (i) the equatorial rotation rate of
the corona is comparable to the rotation rate of the photosphere and the
chromosphere, (ii) the differential profile with respect to the latitude varies
throughout the period of the study; it is more in the year 1999 and least in
1994 and (iii) the equatorial rotation period varies systematically with
sunspot numbers and indicates its dependence on the phases of the solar
activity cycle.Comment: 9 Pages, 4 Figures, Accepted for Publication in MNRA
Physical Properties of Radio Stars Based on LAMOST Spectral Survey
Radio emission has been detected for all types of stars in the Hertzsprung Russell diagram. Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) low and medium-resolution spectroscopic surveys provide a good opportunity to obtain the spectroscopic properties of radio stars. We cross-matched big data from the LAMOST DR7 low resolution spectral survey with a catalogue of radio stars, and obtained 449 stellar spectra of 258 stars. We detected 185 spectra with Hα role= presentation style= box-sizing: border-box; max-height: none; display: inline; line-height: normal; font-size: 13.2px; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; color: rgb(34, 34, 34); font-family: Arial, Arial, Helvetica, sans-serif; position: relative; \u3eα emission. There are a total of 108 objects with repeated low resolution spectral observations, of which 63 show variations in the Hα role= presentation style= box-sizing: border-box; max-height: none; display: inline; line-height: normal; font-size: 13.2px; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; color: rgb(34, 34, 34); font-family: Arial, Arial, Helvetica, sans-serif; position: relative; \u3eα line over a long time, about several months. We cross-matched LAMOST DR7 medium resolution spectra with the radio star catalogue. We obtained 1319 LAMOST medium-resolution spectra of the 156 radio stars and then calculated their equivalent widths (EWs role= presentation style= box-sizing: border-box; max-height: none; display: inline; line-height: normal; font-size: 13.2px; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; color: rgb(34, 34, 34); font-family: Arial, Arial, Helvetica, sans-serif; position: relative; \u3eEWs) of the Hα role= presentation style= box-sizing: border-box; max-height: none; display: inline; line-height: normal; font-size: 13.2px; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; color: rgb(34, 34, 34); font-family: Arial, Arial, Helvetica, sans-serif; position: relative; \u3eα line. Among them, 93 radio stars with Hα role= presentation style= box-sizing: border-box; max-height: none; display: inline; line-height: normal; font-size: 13.2px; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; color: rgb(34, 34, 34); font-family: Arial, Arial, Helvetica, sans-serif; position: relative; \u3eα emission were found from the LAMOST medium resolution spectra, and 63 objects showed short and long-term variabilities, especially on a short time scale of approximately 20 min. Finally, we estimated the minimal detectable radio flux of the FAST telescope and provided a scientific plan for studying radio stars
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