121 research outputs found
THE Hα EMISSION OF NEARBY M DWARFS AND ITS RELATION TO STELLAR ROTATION
The high-energy emission from low-mass stars is mediated by the magnetic dynamo. Although the mechanisms by which fully convective stars generate large-scale magnetic fields are not well understood, it is clear that, as for solar-type stars, stellar rotation plays a pivotal role. We present 270 new optical spectra of low-mass stars in the Solar Neighborhood. Combining our observations with those from the literature, our sample comprises 2202 measurements or non-detections of Hα emission in nearby M dwarfs. This includes 466 with photometric rotation periods. Stars with masses between 0.1 and 0.6 M[subscript ⊙] are well-represented in our sample, with fast and slow rotators of all masses. We observe a threshold in the mass–period plane that separates active and inactive M dwarfs. The threshold coincides with the fast-period edge of the slowly rotating population, at approximately the rotation period at which an era of rapid rotational evolution appears to cease. The well-defined active/inactive boundary indicates that Hα activity is a useful diagnostic for stellar rotation period, e.g., for target selection for exoplanet surveys, and we present a mass-period relation for inactive M dwarfs. We also find a significant, moderate correlation between L[suscript Hα]/L[subscript bol] and variability amplitude: more active stars display higher levels of photometric variability. Consistent with previous work, our data show that rapid rotators maintain a saturated value of LHα/Lbol. Our data also show a clear power-law decay in L[subscript Hα]/L[subscript bol] with Rossby number for slow rotators, with an index of −1.7 ± 0.1.National Science Foundation (U.S.). Astronomy and Astrophysics Postdoctoral Fellowship (Award AST-1602597
Spin orbit alignment for KELT-7b and HAT-P-56b via Doppler tomography with TRES
We present Doppler tomographic analyses for the spectroscopic transits of
KELT-7b and HAT-P-56b, two hot-Jupiters orbiting rapidly rotating F-dwarf host
stars. These include analyses of archival TRES observations for KELT-7b, and a
new TRES transit observation of HAT-P-56b. We report spin-orbit aligned
geometries for KELT-7b (2.7 +/- 0.6 deg) and HAT-P-56b (8 +/- 2 deg). The host
stars KELT-7 and HAT-P-56 are among some of the most rapidly rotating
planet-hosting stars known. We examine the tidal re-alignment model for the
evolution of the spin-orbit angle in the context of the spin rates of these
stars. We find no evidence that the rotation rates of KELT-7 and HAT-P-56 have
been modified by star-planet tidal interactions, suggesting that the spin-orbit
angle of systems around these hot stars may represent their primordial
configuration. In fact, KELT-7 and HAT-P-56 are two of three systems in
super-synchronous, spin-orbit aligned states, where the rotation periods of the
host stars are faster than the orbital periods of the planets.Comment: 9 pages, accepted for publication in MNRA
Optical Spectra of 73 Stripped-Envelope Core-Collapse Supernovae
We present 645 optical spectra of 73 supernovae (SNe) of Types IIb, Ib, Ic,
and broad-lined Ic. All of these types are attributed to the core collapse of
massive stars, with varying degrees of intact H and He envelopes before
explosion. The SNe in our sample have a mean redshift = 4200 km/s. Most of
these spectra were gathered at the Harvard-Smithsonian Center for Astrophysics
(CfA) between 2004 and 2009. For 53 SNe, these are the first published spectra.
The data coverage range from mere identification (1-3 spectra) for a few SNe to
extensive series of observations (10-30 spectra) that trace the spectral
evolution for others, with an average of 9 spectra per SN. For 44 SNe of the 73
SNe presented here, we have well-determined dates of maximum light to determine
the phase of each spectrum. Our sample constitutes the most extensive spectral
library of stripped-envelope SNe to date. We provide very early coverage (as
early as 30 days before V-band max) for photospheric spectra, as well as
late-time nebular coverage when the innermost regions of the SNe are visible
(as late as 2 years after explosion, while for SN1993J, we have data as late as
11.6 years). This data set has homogeneous observations and reductions that
allow us to study the spectroscopic diversity of these classes of stripped SNe
and to compare these to SNe associated with gamma-ray bursts. We undertake
these matters in follow-up papers.Comment: Published by the Astronomical Journal in May 2015. All spectra are
publicly available at the CfA SN archive:
http://www.cfa.harvard.edu/supernova/SNarchive.html . A companion paper on
constructing SNID templates based on these spectra is by Liu & Modjaz (2014)
and the resulting SNID templates are available from the NYU website:
http://cosmo.nyu.edu/SNYU/spectra
Mid-to-Late M Dwarfs Lack Jupiter Analogs
Cold Jovian planets play an important role in sculpting the dynamical
environment in which inner terrestrial planets form. The core accretion model
predicts that giant planets cannot form around low-mass M dwarfs, although this
idea has been challenged by recent planet discoveries. Here, we investigate the
occurrence rate of giant planets around low-mass (0.1-0.3M) M dwarfs.
We monitor a volume-complete, inactive sample of 200 such stars located within
15 parsecs, collecting four high-resolution spectra of each M dwarf over six
years and performing intensive follow-up monitoring of two candidate
radial-velocity variables. We use TRES on the 1.5 m telescope at the Fred
Lawrence Whipple Observatory and CHIRON on the Cerro Tololo Inter-American
Observatory 1.5 m telescope for our primary campaign, and MAROON-X on Gemini
North for high-precision follow-up. We place a 95%-confidence upper limit of
1.5% (68%-confidence limit of 0.57%) on the occurrence of sin1M giant planets out to the water snow line and provide additional
constraints on the giant planet population as a function of sin
and period. Beyond the snow line ( K K), we place
95%-confidence upper limits of 1.5%, 1.7%, and 4.4% (68%-confidence limits of
0.58%, 0.66%, and 1.7%) for 3MsinM,
0.8MsinM, and 0.3MsinM giant planets; i.e., Jupiter analogs are rare around
low-mass M dwarfs. In contrast, surveys of Sun-like stars have found that their
giant planets are most common at these Jupiter-like instellations.Comment: Accepted for publication in AJ; 19 pages, 5 figures, 2 table
LHS 1610A: A Nearby Mid-M Dwarf with a Companion That is Likely A Brown Dwarf
We present the spectroscopic orbit of LHS 1610A, a newly discovered
single-lined spectroscopic binary with a trigonometric distance placing it at
9.9 pm 0.2 pc. We obtained spectra with the TRES instrument on the 1.5m
Tillinghast Reflector at the Fred Lawrence Whipple Observatory located on Mt.
Hopkins in AZ. We demonstrate the use of the TiO molecular bands at 7065 --
7165 Angstroms to measure radial velocities and achieve an average estimated
velocity uncertainty of 28 m/s. We measure the orbital period to be 10.6 days
and calculate a minimum mass of 44.8 pm 3.2 Jupiter masses for the secondary,
indicating that it is likely a brown dwarf. We place an upper limit to 3 sigma
of 2500 K on the effective temperature of the companion from infrared
spectroscopic observations using IGRINS on the 4.3m Discovery Channel
Telescope. In addition, we present a new photometric rotation period of 84.3
days for the primary star using data from the MEarth-South Observatory, with
which we show that the system does not eclipse.Comment: 10 pages, 5 figures; accepted for publication in the Astronomical
Journa
Five Planets Transiting a Ninth Magnitude Star
The Kepler mission has revealed a great diversity of planetary systems and
architectures, but most of the planets discovered by Kepler orbit faint stars.
Using new data from the K2 mission, we present the discovery of a five planet
system transiting a bright (V = 8.9, K = 7.7) star called HIP 41378. HIP 41378
is a slightly metal-poor late F-type star with moderate rotation (v sin(i) = 7
km/s) and lies at a distance of 116 +/- 18 from Earth. We find that HIP 41378
hosts two sub-Neptune sized planets orbiting 3.5% outside a 2:1 period
commensurability in 15.6 and 31.7 day orbits. In addition, we detect three
planets which each transit once during the 75 days spanned by K2 observations.
One planet is Neptune sized in a likely ~160 day orbit, one is sub-Saturn sized
likely in a ~130 day orbit, and one is a Jupiter sized planet in a likely ~1
year orbit. We show that these estimates for the orbital periods can be made
more precise by taking into account dynamical stability considerations. We also
calculate the distribution of stellar reflex velocities expected for this
system, and show that it provides a good target for future radial velocity
observations. If a precise orbital period can be determined for the outer
Jovian planet through future observations, it will be an excellent candidate
for follow-up transit observations to study its atmosphere and measure its
oblateness.Comment: Accepted by ApJL. 12 pages, 6 figures, 2 table
Discovery and Precise Characterization by the MEarth Project of LP 661-13, an Eclipsing Binary Consisting of Two Fully Convective Low-mass Stars
We report the detection of stellar eclipses in the LP 661-13 system. We
present the discovery and characterization of this system, including high
resolution spectroscopic radial velocities and a photometric solution spanning
two observing seasons. LP 661-13 is a low mass binary system with an orbital
period of days at a distance of parsecs. LP 661-13A is a star while LP
661-13B is a star. The radius of each component
is and ,
respectively. We detect out of eclipse modulations at a period slightly shorter
than the orbital period, implying that at least one of the components is not
rotating synchronously. We find that each component is slightly inflated
compared to stellar models, and that this cannot be reconciled through age or
metallicity effects. As a nearby eclipsing binary system where both components
are near or below the full-convection limit, LP 661-13 will be a valuable test
of models for the structure of cool dwarf stars.Comment: 24 pages, 8 tables, 6 figures. Submitted to ApJ, comments welcom
The CIDA Variability Survey of Orion OB1. I: the low-mass population of Ori OB 1a and 1b
We present results of a large scale, multi-epoch optical survey of the Ori
OB1 association, carried out with the QuEST camera at the Venezuela National
Astronomical Observatory. We identify for the first time the widely spread
low-mass, young population in the Orion OB1a and OB1b sub-associations.
Candidate members were picked up by their variability in the V-band and
position in color-magnitude diagrams. We obtained spectra to confirm
membership. In a region spanning ~ 68 deg^2 we found 197 new young stars; of
these, 56 are located in the Ori OB1a subassociation and 142 in Ori OB1b.
Comparison with the spatial extent of molecular gas and extinction maps
indicates that the subassociation Ori 1b is concentrated within a ring-like
structure of radius ~2 deg (~15 pc at 440 pc), centered roughly on the star
epsilon Ori in the Orion belt. The ring is apparent in 13CO and corresponds to
a region with an extinction Av>=1. The stars exhibiting strong Ha emission, an
indicator of active accretion, are found along this ring, while the center is
populated with weak Ha emitting stars. In contrast, Ori OB1a is located in a
region devoid of gas and dust. We identify a grouping of stars within a ~3
deg^2 area located in 1a, roughly clustered around the B2 star 25 Ori. The
Herbig Ae/Be star V346 Ori is also associated with this grouping, which could
be an older analog of sigma Ori. Using using several sets of evolutionary
tracks we find an age of 7 - 10 Myr for Ori 1a and of ~4 - 6 Myr for Ori OB1b,
consistent with previous estimates from OB stars. Indicators such as the
equivalent width of Ha and near-IR excesses show that while a substantial
fraction of accreting disks remain at ages ~5 Myr, inner disks are essentially
dissipated by 10 Myr.Comment: 44 pages, 12 figures, to appear in the Astronomical Journal.
(Abridged abstract - to fit length limit in astroph) Full resolution figures
in http://www.cida.ve/~briceno/publications
Type IIb Supernova SN 2011dh: Spectra and Photometry from the Ultraviolet to the Near-Infrared
We report spectroscopic and photometric observations of the Type IIb SN
2011dh obtained between 4 and 34 days after the estimated date of explosion
(May 31.5 UT). The data cover a wide wavelength range from 2,000 Angstroms in
the UV to 2.4 microns in the NIR. Optical spectra provide line profiles and
velocity measurements of HI, HeI, CaII and FeII that trace the composition and
kinematics of the SN. NIR spectra show that helium is present in the atmosphere
as early as 11 days after the explosion. A UV spectrum obtained with the STIS
reveals that the UV flux for SN 2011dh is low compared to other SN IIb. The HI
and HeI velocities in SN 2011dh are separated by about 4,000 km/s at all
phases. We estimate that the H-shell of SN 2011dh is about 8 times less massive
than the shell of SN 1993J and about 3 times more massive than the shell of SN
2008ax. Light curves (LC) for twelve passbands are presented. The maximum
bolometric luminosity of erg s occurred
about 22 days after the explosion. NIR emission provides more than 30% of the
total bolometric flux at the beginning of our observations and increases to
nearly 50% of the total by day 34. The UV produces 16% of the total flux on day
4, 5% on day 9 and 1% on day 34. We compare the bolometric light curves of SN
2011dh, SN 2008ax and SN 1993J. The LC are very different for the first twelve
days after the explosions but all three SN IIb display similar peak
luminosities, times of peak, decline rates and colors after maximum. This
suggests that the progenitors of these SN IIb may have had similar compositions
and masses but they exploded inside hydrogen shells that that have a wide range
of masses. The detailed observations presented here will help evaluate
theoretical models for this supernova and lead to a better understanding of SN
IIb.Comment: 23 pages, 14 figures, 9 tables, accepted by Ap
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