49 research outputs found
Most Rotational Variables Dominated by a Single Bright Feature are CVn Stars
We previously reported a rare class of variable star light curves isolated
from a sample of 4.7 million candidate variables from the ATLAS survey. Dubbed
`UCBH' light curves, they have broad minima and narrow, symmetrical maxima,
with typical periods of 1-10 days and amplitudes of 0.05--0.20 mag. They
maintain constant amplitude, shape, and phase coherence over multiple years,
but do not match any known class of pulsating variables. A localized bright
spot near the equator of a rotating star will produce a UCBH-type light curve
for most viewing geometries. Most stars that exhibit rotational variability
caused primarily by a single bright feature should therefore appear as UCBH
stars, although a rotating bright spot is not the only thing that could produce
a UCBH-type lightcurve. We have spectroscopically investigated fourteen UCBH
stars and found ten of them to be Ap/Bp stars: A-type or B-type stars with
greatly enhanced photospheric abundances of specific heavy elements.
Rotationally variable Ap/Bp stars are referred to as CVn variables.
Most ATLAS UCBH stars are therefore CVn stars, although only a
minority of CVn stars in the literature have UCBH light curves. The
fact that CVn stars dominate the UCBH class suggests that lone
bright spots with sufficient size and contrast develop more readily on Ap/Bp
stars than on any other type. The CVn UCBH stars may be
characterized by a specific magnetic field topology, making them intriguing
targets for future Zeeman-Doppler imaging.Comment: 18 pages, 8 figures, accepted to A
Brown Dwarfs in Young Moving Groups from Pan-STARRS1. I. AB Doradus
Substellar members of young (150 Myr) moving groups are valuable
benchmarks to empirically define brown dwarf evolution with age and to study
the low-mass end of the initial mass function. We have combined Pan-STARRS1
(PS1) proper motions with opticalIR photometry from PS1, 2MASS and
to search for substellar members of the AB Dor Moving Group
within 50 pc and with spectral types of late-M to early-L,
corresponding to masses down to 30 M at the age of the group
(125 Myr). Including both photometry and proper motions allows us to
better select candidates by excluding field dwarfs whose colors are similar to
young AB~Dor Moving Group members. Our near-IR spectroscopy has identified six
ultracool dwarfs (M6L4; 30100 M) with intermediate
surface gravities (INT-G) as candidate members of the AB Dor Moving Group. We
find another two candidate members with spectra showing hints of youth but
consistent with field gravities. We also find four field brown dwarfs
unassociated with the AB Dor Moving Group, three of which have INT-G gravity
classification. While signatures of youth are present in the spectra of our
125 Myr objects, neither their nor colors are
significantly redder than field dwarfs with the same spectral types, unlike
younger ultracool dwarfs. We also determined PS1 parallaxes for eight of our
candidates and one previously identified AB Dor Moving Group candidate.
Although radial velocities (and parallaxes, for some) are still needed to fully
assess membership, these new objects provide valuable insight into the spectral
characteristics and evolution of young brown dwarfs.Comment: ApJ, accepte
A First Catalog of Variable Stars Measured by the Asteroid Terrestrial-impact Last Alert System (ATLAS)
The Asteroid Terrestrial-impact Last Alert System (ATLAS) carries out its
primary planetary defense mission by surveying about 13000 deg^2 at least four
times per night. The resulting data set is useful for the discovery of variable
stars to a magnitude limit fainter than r~18, with amplitudes down to 0.01 mag
for bright objects. Here we present a Data Release One catalog of variable
stars based on analyzing 142 million stars measured at least 100 times in the
first two years of ATLAS operations. Using a Lomb-Scargle periodogram and other
variability metrics, we identify 4.7 million candidate variables which we
analyze in detail. Through Space Telescope Science Institute, we publicly
release lightcurves for all of them, together with a vector of 169
classification features for each star. We do this at the level of unconfirmed
candidate variables in order to provide the community with a large set of
homogeneously analyzed photometry and avoid pre-judging which types of objects
others may find most interesting. We use machine learning to classify the
candidates into fifteen different broad categories based on lightcurve
morphology. About 10% (430,000 stars) pass extensive tests designed to screen
out spurious variability detections: we label these as `probable' variables. Of
these, 230,000 receive specific classifications as eclipsing binaries,
pulsating, Mira-type, or sinusoidal variables: these are the `classified'
variables. New discoveries among the probable variables number more than
300,000, while 150,000 of the classified variables are new, including about
10,000 pulsating variables, 2,000 Mira stars, and 70,000 eclipsing binaries.Comment: Accepted by AJ; gives instructions for querying ATLAS variable star
database; this new version has nicer lightcurve figure
The splitting of double-component active asteroid P/2016 J1 (PANSTARRS)
We present deep imaging observations, orbital dynamics, and dust tail model
analyses of the double-component asteroid P/2016 J1 (J1-A and J1-B). The
observations were acquired at the Gran Telescopio Canarias (GTC) and the
Canada-France-Hawaii Telescope (CFHT) from mid March to late July, 2016. A
statistical analysis of backward-in-time integrations of the orbits of a large
sample of clone objects of P/2016 J1-A and J1-B shows that the minimum
separation between them occurred most likely 2300 days prior to the
current perihelion passage, i.e., during the previous orbit near perihelion.
This closest approach was probably linked to a fragmentation event of their
parent body. Monte Carlo dust tail models show that those two components became
active simultaneously 250 days before the current perihelion, with
comparable maximum loss rates of 0.7 kg s and 0.5 kg
s, and total ejected masses of 810 kg and
610 kg for fragments J1-A and J1-B, respectively. In consequence,
the fragmentation event and the present dust activity are unrelated. The
simultaneous activation times of the two components and the fact that the
activity lasted 6 to 9 months or longer, strongly indicate ice sublimation as
the most likely mechanism involved in the dust emission process.Comment: Accepted by ApJ Letters, Feb. 17, 201