5,576 research outputs found
OB stars at the lowest Local Group metallicity: GTC-OSIRIS observations of Sextans A
Our aim is to find and classify OB stars in Sextans A, to later determine
accurate stellar parameters of these blue massive stars in this low metallicity
region .
Using UBV photometry, the reddening-free index Q and GALEX imaging, we built
a list of blue massive star candidates in Sextans A. We obtained low resolution
(R 1000) GTC-OSIRIS spectra for a fraction of them and carried out
spectral classification. For the confirmed O-stars we derive preliminary
stellar parameters.
The target selection criteria and observations were successful and have
produced the first spectroscopic atlas of OB-type stars in Sextans A. From the
whole sample of 18 observed stars, 12 were classified as early OB-types,
including 5 O-stars. The radial velocities of all target stars are in agreement
with their Sextans A membership, although three of them show significant
deviations. We determined the stellar parameters of the O-type stars using the
stellar atmosphere code FASTWIND, and revisited the sub-SMC temperature scale.
Two of the O-stars are consistent with relatively strong winds and enhanced
helium abundances, although results are not conclusive. We discuss the position
of the OB stars in the HRD. Initial stellar masses run from slightly below 20
up to 40 solar masses.
The target selection method worked well for Sextans A, confirming the
procedure developed in Garcia \& Herrero (2013). The stellar temperatures are
consistent with findings in other galaxies. Some of the targets deserve
follow-up spectroscopy because of indications of a runaway nature, an enhanced
helium abundance or a relatively strong wind. We observe a correlation between
HI and OB associations similar to the irregular galaxy IC1613, confirming the
previous result that the most recent star formation of Sextans A is currently
on-going near the rim of the H\,{\sc I} cavity
Electronic Transport in Dual-gated Bilayer Graphene at Large Displacement Fields
We study the electronic transport properties of dual-gated bilayer graphene
devices. We focus on the regime of low temperatures and high electric
displacement fields, where we observe a clear exponential dependence of the
resistance as a function of displacement field and density, accompanied by a
strong non-linear behavior in the transport characteristics. The effective
transport gap is typically two orders of magnitude smaller than the optical
band gaps reported by infrared spectroscopy studies. Detailed temperature
dependence measurements shed light on the different transport mechanisms in
different temperature regimes.Comment: 4 pages, 3 figure
Localització d'emplaçaments per a noves estacions o canvi d'ubicació d'estacions existents en línies de ferrocarril
La xarxa ferroviària catalana està basada en un seguit d'infraestructures que tenen el seu origen a mitjans o finals del segle XIX, les quals han mantingut la seva configuració fins l'actualitat. El creixement urbanístic registrat dins la perifèria de la ciutat de Barcelona ha fet, però, que les necessitats de la població hagin variat, fent necessari un servei ràpid i de calitat que arribi al major nombre d'usuaris possible, el que fa plantejar l'aparició de nous punts d'accés a la xarxa. Així doncs, a partir de dades d'assentament de població, mobilitat i creixement urbanístic es podran buscar nous emplaçaments d'estacions o modificacions dels ja existents amb l'objectiu de millorar el servei ofert
Photospheric activity, rotation and magnetic interaction in LHS 6343 A
Context. The Kepler mission has recently discovered a brown dwarf companion
transiting one member of the M4V+M5V visual binary system LHS 6343 AB with an
orbital period of 12.71 days. Aims. The particular interest of this transiting
system lies in the synchronicity between the transits of the brown dwarf C
component and the main modulation observed in the light curve, which is assumed
to be caused by rotating starspots on the A component. We model the activity of
this star by deriving maps of the active regions that allow us to study stellar
rotation and the possible interaction with the brown dwarf companion. Methods.
An average transit profile was derived, and the photometric perturbations due
to spots occulted during transits are removed to derive more precise transit
parameters. We applied a maximum entropy spot model to fit the out-of-transit
optical modulation as observed by Kepler during an uninterrupted interval of
500 days. It assumes that stellar active regions consist of cool spots and
bright faculae whose visibility is modulated by stellar rotation. Results.
Thanks to the extended photometric time series, we refine the determination of
the transit parameters and find evidence of spots that are occulted by the
brown dwarf during its transits. The modelling of the out-of-transit light
curve of LHS 6343 A reveals several starspots rotating with a slightly longer
period than the orbital period of the brown dwarf, i.e., 13.13 +- 0.02 days. No
signature attributable to differential rotation is observed. We find evidence
of a persistent active longitude on the M dwarf preceding the sub- companion
point by 100 deg and lasting for at least 500 days. This can be relevant for
understanding how magnetic interaction works in low-mass binary and star-planet
systems.Comment: 14 pages, 16 figure
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