230 research outputs found
Precision photometric monitoring of very low mass Sigma Orionis cluster members: variability and rotation at a few Myr
We present high-precision photometry on 107 variable low-mass stars and brown
dwarfs in the ~3 Myr Sigma Orionis open cluster. We have carried out I-band
photometric monitoring within two fields, encompassing 153 confirmed or
candidate members of the low-mass cluster population, from 0.02 to 0.5 M_\odot.
We are sensitive to brightness changes on time scales from 10 minutes to two
weeks with amplitudes as low as 0.004 magnitudes, and find variability on these
time scales in nearly 70% of cluster members. We identify both periodic and
aperiodic modes of variability, as well as semi-periodic rapid fading events
that are not accounted for by the standard explanations of rotational
modulation of surface features or accretion. We have incorporated both optical
and infrared color data to uncover trends in variability with mass and
circumstellar disks. While the data confirm that the lowest-mass objects
(M<0.2M_\odot) rotate more rapidly than the 0.2-0.5 M_\odot members, they do
not support a direct connection between rotation rate and the presence of a
disk. Finally, we speculate on the origin of irregular variability in cluster
members with no evidence for disks or accretion.Comment: 40 pages, 18 figures, accepted for publication in ApJS. Link to
electronic figures correcte
The Many-faceted Light Curves of Young Disk-bearing Stars in Upper Sco –– Oph Observed by K2 Campaign 2
The K2 Mission has photometrically monitored thousands of stars at high precision and cadence in a series of ~80-day campaigns focused on sections of the ecliptic plane. During its second campaign, K2 targeted over 1000 young stellar objects (YSOs) in the ~1–3 Myr ρ Ophiuchus and 5–10 Myr Upper Scorpius regions. From this set, we have carefully vetted photometry from WISE and Spitzer to identify those YSOs with infrared excess indicative of primordial circumstellar disks. We present here the resulting comprehensive sample of 288 young disk-bearing stars from B through M spectral types and analysis of their associated K2 light curves. Using statistics of periodicity and symmetry, we categorize each light curve into eight different variability classes, notably including "dippers" (fading events), "bursters" (brightening events), stochastic, and quasi-periodic types. Nearly all (96%) of disk-bearing YSOs are identified as variable at 30-minute cadence with the sub-1% precision of K2. Combining our variability classifications with (circum)stellar properties, we find that the bursters, stochastic sources, and the largest amplitude quasi-periodic stars have larger infrared colors, and hence stronger circumstellar disks. They also tend to have larger Hα equivalent widths, indicative of higher accretion rates. The dippers, on the other hand, cluster toward moderate infrared colors and low Hα. Using resolved disk observations, we further find that the latter favor high inclinations, except for a few notable exceptions with close to face-on disks. These observations support the idea that YSO time-domain properties are dependent on several factors, including accretion rate and view angle
A Pulsation Search Among Young Brown Dwarfs and Very Low Mass Stars
In 2005, Palla & Baraffe proposed that brown dwarfs (BDs) and very low mass
stars (VLMSs; <0.1 solar masses) may be unstable to radial oscillations during
the pre-main-sequence deuterium burning phase. With associated periods of 1-4
hours, this potentially new class of pulsation offers unprecedented
opportunities to probe the interiors and evolution of low-mass objects in the
1-15 million year age range. Following up on reports of short-period
variability in young clusters, we designed a high-cadence photometric
monitoring campaign to search for deuterium-burning pulsation among a sample of
348 BDs and VLMSs in the four young clusters Orionis, Chamaeleon I, IC
348, and Upper Scorpius. In the resulting light curves we achieved sensitivity
to periodic signals of amplitude several millimagnitudes, on timescales from 15
minutes to two weeks. Despite the exquisite data quality, we failed to detect
any periodicities below seven hours. We conclude that D-burning pulsations are
not able to grow to observable amplitudes in the early pre-main sequence. In
spite of the non-detection, we did uncover a rich set of variability behavior-
both periodic and aperiodic- on day to week timescales. We present new
compilations of variable sources from our sample, as well as three new
candidate cluster members in Chamaeleon I.Comment: 21 pages, 15 figures; accepted for publication in Ap
Pulsation Powered by Deuterium Burning in Brown Dwarfs and Very-Low-Mass Stars
Pulsation powered by deuterium burning in brown dwarfs and very low mass stars has been put forth (Palla & Baraffe 2005) as a novel probe of the interiors of these objects in the 1–15 Myr age range. Previous observations have hinted at variability on the expected timescales of a few hours, suggesting but not confirming that the phenomenon is at work in young brown dwarfs. We have recently carried out a dedicated campaign to search for this putative class of pulsators among known low-mass members of five young star clusters. Our survey achieves sensitivity to periodic oscillations with photometric amplitudes down to several millimagnitudes. We present the census of variability over timescales ranging from minutes to days and discuss the current prospects for pulsation as a tool in the study of young, objects near the substellar boundary. As a byproduct, this work provides new insights into the distribution of stellar rotation periods at young ages via the detection of variability due to cool surface spots
Simulated Performance of Timescale Metrics for Aperiodic Light Curves
Aperiodic variability is a characteristic feature of young stars, massive
stars, and active galactic nuclei. With the recent proliferation of time domain
surveys, it is increasingly essential to develop methods to quantify and
analyze aperiodic variability. We develop three timescale metrics that have
been little used in astronomy -- {\Delta}m-{\Delta}t plots, peak-finding, and
Gaussian process regression -- and present simulations comparing their
effectiveness across a range of aperiodic light curve shapes, characteristic
timescales, observing cadences, and signal to noise ratios. We find that
Gaussian process regression is easily confused by noise and by irregular
sampling, even when the model being fit reflects the process underlying the
light curve, but that {\Delta}m-{\Delta}t plots and peak-finding can coarsely
characterize timescales across a broad region of parameter space. We make
public the software we used for our simulations, both in the spirit of open
research and to allow others to carry out analogous simulations for their own
observing programs.Comment: 39 pages, 16 figures, 3 tables. To be published in The Astrophysical
Journal. Simulation software is available online at http://ascl.net/1408.01
A Search for Pulsation in Very Low-mass Stars and Brown Dwarfs
Brown dwarfs and very low-mass stars constitute a crucial link between the intertwined processes of star formation and planet formation. To date, however, observational methods to uncover their formation mechanism or determine important properties such as mass and age have been lacking. Pulsation powered by deuterium burning in brown dwarfs and very low-mass stars is a newly suggested phenomenon that offers unprecedented opportunities to probe the interiors and evolution of these objects. We report on a photometric campaign to search for low-amplitude pulsations among young star-cluster members using a number of telescopes
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