49 research outputs found
Pulsed Accretion in the T Tauri Binary TWA 3A
TWA 3A is the most recent addition to a small group of young binary systems
that both actively accrete from a circumbinary disk and have spectroscopic
orbital solutions. As such, it provides a unique opportunity to test binary
accretion theory in a well-constrained setting. To examine TWA 3A's
time-variable accretion behavior, we have conducted a two-year, optical
photometric monitoring campaign, obtaining dense orbital phase coverage (~20
observations per orbit) for ~15 orbital periods. From U-band measurements we
derive the time-dependent binary mass accretion rate, finding bursts of
accretion near each periastron passage. On average, these enhanced accretion
events evolve over orbital phases 0.85 to 1.05, reaching their peak at
periastron. The specific accretion rate increases above the quiescent value by
a factor of ~4 on average but the peak can be as high as an order of magnitude
in a given orbit. The phase dependence and amplitude of TWA 3A accretion is in
good agreement with numerical simulations of binary accretion with similar
orbital parameters. In these simulations, periastron accretion bursts are
fueled by periodic streams of material from the circumbinary disk that are
driven by the binary orbit. We find that TWA 3A's average accretion behavior is
remarkably similar to DQ Tau, another T Tauri binary with similar orbital
parameters, but with significantly less variability from orbit to orbit. This
is only the second clear case of orbital-phase-dependent accretion in a T Tauri
binary.Comment: 6 pages, 4 figure
X-Ray Grating Observations of Recurrent Nova T Pyxidis During The 2011 Outburst
The recurrent nova T Pyx was observed with the X-ray gratings of Chandra and
XMM-Newton, 210 and 235 days, respectively, after the discovery of the 2011
April 14 outburst. The X-ray spectra show prominent emission lines of C, N, and
O, with broadening corresponding to a full width at half maximum of ~2000-3000
km/s, and line ratios consistent with high-density plasma in collisional
ionization equilibrium. On day 210 we also measured soft X-ray continuum
emission that appears to be consistent with a white dwarf (WD) atmosphere at a
temperature ~420,000 K, partially obscured by anisotropic, optically thick
ejecta. The X-ray continuum emission is modulated with the photometric and
spectroscopic period observed in quiescence. The continuum at day 235 indicated
a WD atmosphere at a consistent effective temperature of 25 days earlier, but
with a lower flux. The effective temperature indicates a mass of ~1 solar mass.
The conclusion of partial WD obscuration is supported by the complex geometry
of non-spherically-symmetric ejecta confirmed in recent optical spectra
obtained with the Southern African Large Telescope (SALT) in November and
December of 2012. These spectra exhibited prominent [O III] nebular lines with
velocity structures typical of bipolar ejecta.Comment: Accepted to ApJ 2013 October 23, 14 pages, 9 figures, 3 table
SPYGLASS. II. The Multi-Generational and Multi-Origin Star Formation History of Cepheus Far North
Young stellar populations provide a record of past star formation, and by
establishing their members' dynamics and ages, it is possible to reconstruct
the full history of star formation events. Gaia has greatly expanded the number
of accessible stellar populations, with one of the most notable
recently-discovered associations being Cepheus Far North (CFN), a population
containing hundreds of members spanning over 100 pc. With its proximity (d
200 pc), apparent substructure, and relatively small population, CFN
represents a manageable population to study in depth, with enough evidence of
internal complexity to produce a compelling star formation story. Using Gaia
astrometry and photometry combined with additional spectroscopic observations,
we identify over 500 candidate CFN members spread across 7 subgroups. Combining
ages from isochrones, asteroseismology, dynamics, and lithium depletion, we
produce well-constrained ages for all seven subgroups, revealing a largely
continuous 10 Myr star formation history in the association. By tracing back
the present-day populations to the time of their formation, we identify two
spatially and dynamically distinct nodes in which stars form, one associated
with Cephei which shows mostly co-spatial formation, and one associated
with EE Draconis with a more dispersed star formation history. This detailed
view of star formation demonstrates the complexity of the star formation
process, even in the smallest of regions.Comment: Accepted to ApJ; 34 pages, 15 figures, 6 tables in two-column
AASTEX63 forma
Correlating Changes in Spot Filling Factors with Stellar Rotation: The Case of LkCa 4
We present a multi-epoch spectroscopic study of LkCa 4, a heavily spotted
non-accreting T Tauri star. Using SpeX at NASA's Infrared Telescope Facility
(IRTF), 12 spectra were collected over five consecutive nights, spanning
1.5 stellar rotations. Using the IRTF SpeX Spectral Library, we
constructed empirical composite models of spotted stars by combining a warmer
(photosphere) standard star spectrum with a cooler (spot) standard weighted by
the spot filling factor, . The best-fit models spanned two
photospheric component temperatures, = 4100 K (K7V) and 4400 K
(K5V), and one spot component temperature, = 3060 K (M5V) with an
of 0.3. We find values of to vary between 0.77 and 0.94 with
an average uncertainty of 0.04. The variability of is periodic
and correlates with its 3.374 day rotational period. Using a mean value for
to represent the total spot coverage, we calculated spot
corrected values for and . Placing these values alongside
evolutionary models developed for heavily spotted young stars, we infer mass
and age ranges of 0.45-0.6 and 0.50-1.25 Myr, respectively. These
inferred values represent a twofold increase in the mass and a twofold decrease
in the age as compared to standard evolutionary models. Such a result
highlights the need for constraining the contributions of cool and warm regions
of young stellar atmospheres when estimating and to infer
masses and ages as well as the necessity for models to account for the effects
of these regions on the early evolution of low-mass stars.Comment: 21 pages, 9 Figures; Accepted for publication in Ap
A Lithium Depletion Age for the Carina Association
The dispersed remnants of stellar nurseries, stellar associations provide
unparalleled samples of coeval stars critical for studies of stellar and
planetary formation and evolution. The Carina Stellar Association is one of the
closest stellar associations to Earth, and yet measurements of its age have
varied from 13 to 45 Myr. We aim to update the age of Carina using the Lithium
Depletion Boundary method. We obtain new measurements of the Li 6708 Angstrom,
absorption feature in likely members using optical spectra from the Goodman HTS
on SOAR and NRES on LCO. We detect the depletion boundary at M_K ~= 6.8 (M5),
which corresponds to an age of 41(+3,-5) Myr. The age is consistent within
uncertainties across six different models, including those that account for
magnetic fields and spots. We also estimate the age through analysis of the
group's overall variability, and by comparing the association members' CMD to
stellar evolutionary models using a Gaussian Mixture Model, recovering ages
consistent with the LDB. The resulting age agrees with the older end of
previous age measurements and is consistent with the lithium depletion age for
the neighboring Tucana-Horologium Moving Group.Comment: 9 pages, 6 figures, accepted to AJ on 10/17/202