816 research outputs found
Hardware and software identification of communication channels based on the Volterra model in frequency domain
Surprisingly different star-spot distributions on the near equal-mass equal-rotation-rate stars in the M dwarf binary GJ 65 AB
We aim to understand how stellar parameters such as mass and rotation impact the distribution of star-spots on the stellar surface. To this purpose, we have used Doppler imaging to reconstruct the surface brightness distributions of three fully convective M dwarfs with similar rotation rates. We secured high cadence spectral time series observations of the 5.5 au separation binary GJ 65, comprising GJ 65A (M5.5V, Prot = 0.24 d) and GJ 65B (M6V, Prot = 0.23 d). We also present new observations of GJ 791.2A (M4.5V, Prot = 0.31 d). Observations of each star were made on two nights with UVES, covering a wavelength range from 0.64 - 1.03μm. The time series spectra reveal multiple line distortions that we interpret as cool star-spots and which are persistent on both nights suggesting stability on the time-scale of 3 d. Spots are recovered with resolutions down to 8.3° at the equator. The global spot distributions for GJ 791.2A are similar to observations made a year earlier. Similar high latitude and circumpolar spot structure is seen on GJ 791.2A and GJ 65A. However, they are surprisingly absent on GJ 65B, which instead reveals more extensive, larger, spots concentrated at intermediate latitudes. All three stars show small amplitude latitude-dependent rotation that is consistent with solid body rotation. We compare our measurements of differential rotation with previous Doppler imaging studies and discuss the results in the wider context of other observational estimates and recent theoretical predictions
Atomic lines in infrared spectra for ultracool dwarfs
We provide a set of atomic lines which are suitable for the description of
ultracool dwarf spectra from 10000 to 25000 \AA. This atomic linelist was made
using both synthetic spectra calculations and existing atlases of infrared
spectra of Arcturus and Sunspot umbra. We present plots, which show the
comparison of synthetic spectra and observed Arcturus and Sunspot umbral
spectra for all atomic lines likely to be observable in high resolution
infrared spectra.Comment: 21 pages, 2 tables, 129 figures, figures are available only at
http://www.astro.livjm.ac.uk/~hraj/spectralatlas/index.html, accepted to A&
VLT X-shooter spectroscopy of the nearest brown dwarf binary
The aim of the project is to characterise both components of the nearest
brown dwarf sytem to the Sun, WISE J104915.57-531906.1 (=Luhman16AB) at optical
and near-infrared wavelengths. We obtained high signal-to-noise
intermediate-resolution (R~6000-11000) optical (600-1000 nm) and near-infrared
(1000-2480nm) spectra of each component of Luhman16AB, the closest brown dwarf
binary to the Sun, with the X-Shooter instrument on the Very Large Telescope.
We classify the primary and secondary of the Luhman16 system as L6-L7.5 and
T0+/-1, respectively, in agreement with previous measurements published in the
literature. We present measurements of the lithium pseudo-equivalent widths,
which appears of similar strength on both components (8.2+/-1.0 Angstroms and
8.4+/-1.5 Angstroms for the L and T components, respectively). The presence of
lithium (Lithium 7) in both components imply masses below 0.06 Msun while
comparison with models suggests lower limits of 0.04 Msun. The detection of
lithium in the T component is the first of its kind. Similarly, we assess the
strength of other alkali lines (e.g. pseudo-equivalent widths of 6-7 Angstroms
for RbI and 4-7 Angstroms for CsI) present in the optical and near-infrared
regions and compare with estimates for L and T dwarfs. We also derive effective
temperatures and luminosities of each component of the binary: -4.66+/-0.08 dex
and 1305(+180)(-135) for the L dwarf and -4.68+/-0.13 dex and 1320(+185)(-135)
for the T dwarf, respectively. Using our radial velocity determinations, the
binary does not appear to belong to any of the well-known moving group. Our
preliminary theoretical analysis of the optical and J-band spectra indicates
that the L- and T-type spectra can be reproduced with a single temperature and
gravity but different relative chemical abundances which impact strongly the
spectral energy distribution of L/T transition objects.Comment: 12 pages, 9 figure, 3 tables, accepted to A&
Modeling the spectrum of V4334 Sgr (Sakurai's Object)
Theoretical spectral energy distributions were computed for a grid of
hydrogen-deficient and carbon-rich model atmospheres of T(eff) in the range of
5000-6250 K and log g = 1.0 - 0.0 by the technique of opacity sampling, taking
into account continuous, molecular band and atomic line absorption. These
energy distributions were compared with the spectrum of V4334 Sgr (Sakurai's
object) of April, 1997 in the wavelength interval 300-1000 nm. We show that (1)
the shape of the theoretical spectra depends strongly on T(eff) but only very
weakly on the hydrogen abundance; (2) the comparison of the observed and
computed spectra permits to estimate T(eff) approximately 5500 K for V4334 Sgr
in April, 1997, and its interstellar reddening (plus a possible circumstellar
contribution) E(B-V) approximately 0.70.Comment: 7 pages, 8 figures, LaTeX, accepted by Astronomy and Astrophysic
Masses, Oxygen and Carbon abundances in CHEPS dwarf stars
Reproduced with permission from Astronomy & Astrophysics. © 2019 ESOContext. We report the results from the determination of stellar masses, carbon, and oxygen abundances in the atmospheres of 107 stars from the Calan-Hertfordshire Extrasolar Planet Search (CHEPS) programme. Our stars are drawn from a population with a significantly super-solar metallicity. At least 10 of these stars are known to host orbiting planets. Aims. In this work, we set out to understand the behaviour of carbon and oxygen abundance in stars with different spectral classes, metallicities, and V sin i within the metal-rich stellar population. Methods. Masses of these stars were determined using data from Gaia DR2. Oxygen and carbon abundances were determined by fitting the absorption lines. We determined oxygen abundances with fits to the 6300.304 Å O I line, and we used 3 lines of the C I atom and 12 lines of the C 2 molecule for the determination of carbon abundances. Results. We determine masses and abundances of 107 CHEPS stars. There is no evidence that the [C/O] ratio depends on V sin i or the mass of the star within our constrained range of masses, i.e. 0.82 5 km s -1) are massive stars.Peer reviewedFinal Published versio
Temporal changes of the flare activity of Proxima Cen
We study temporal variations of the emission lines of Halpha, Hepsilon, H and
K Ca II, D1 and D2 Na I, 4026 and 5876 A He I in the HARPS spectra of Proxima
Centauri across an extended time of 13.2 years, from May 27, 2004, to September
30, 2017.
Aims. We analyse the common behaviour and differences in the intensities and
profiles of different emission lines in flare and quiet modes of Proxima
activity.
Methods. We compare the pseudo-equivalent widths (pEW) and profiles of the
emission lines in the HARPS high-resolution (R ~ 115,000) spectra observed at
the same epochs.
Results. All emission lines show variability with a timescale of at least 10
min. The strength of all lines except He I 4026 A correlate with \Halpha.
During strong flares the `red asymmetry' appears in the Halpha emission line
indicating the infall of hot condensed matter into the chromosphere with
velocities greater than 100 km/s disturbing chromospheric layers. As a result,
the strength of the Ca II lines anti-correlates with Halpha during strong
flares. The He I lines at 4026 and 5876 A appear in the strong flares. The
cores of D1 and D2 Na I lines are also seen in emission. During the minimum
activity of Proxima Centauri, Ca II lines and Hepsilon almost disappear while
the blue part of the Na I emission lines is affected by the absorption in the
extending and condensing flows.
Conclusions. We see different behaviour of emission lines formed in the flare
regions and chromosphere. Chromosphere layers of Proxima Cen are likely heated
by the flare events; these layers are cooled in the `non-flare' mode. The
self-absorption structures in cores of our emission lines vary with time due to
the presence of a complicated system of inward and outward matter flows in the
absorbing layers.Comment: 22 pages, 12 Figures, accepted by A
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