Aleks Scholz

Temperaments of young stars: rapid mass accretion rate changes in T Tauri and Herbig Ae stars

Variability in emission lines is a characteristic feature in young stars and can be used as a tool to study the physics of the accretion process. Here, we present a study of Hα variability in 15 T Tauri and Herbig Ae stars (K7 - B2) over a wide range of time windows, from minutes, to hours, to days, and years. We assess the variability using linewidth measurements and the time series of line profiles. All objects show gradual, slow profile changes on time-scales of days. In addition, in three cases there is evidence for rapid variations in Hα with typical timescales of 10 min, which occurs in 10 per cent of the total covered observing time. The mean accretion rate changes, inferred from the linefluxes, are 0.01–0.07 dex for time-scales of <1h, 0.04–0.4 dex for time-scales of days, and 0.13–0.52 dex for time-scales of years. In Costigan et al., we derived an upper limit finding that the intermediate (days) variability dominated over longer (years) variability. Here, our new results, based on much higher cadence observations, also provide a lower limit to accretion rate variability on similar time-scales (days), thereby constraining the accretion rate variability physics in a much more definitive way. A plausible explanation for the gradual variations over days is an asymmetric accretion flow resulting in a rotational modulation of the accretion-related emission, although other interpretations are possible as well. In conjunction with our previous work, we find that the time-scales and the extent of the variability is similar for objects ranging in mass from ∼0.1 to ∼5M� .T his confirms that a single mode of accretion is at work from T Tauri to Herbig Ae stars – across a wide range of stellar masses.

Angular Momentum Evolution of Young Low-Mass Stars and Brown Dwarfs: Observations and Theory

Book chapter published in Protostars & Planets VI (2014) eds. H. Beuther, R. Klessen, K. Dullemond, Th. Henning. University of Arizona Press, ISBN 978-0-8165-3124-0

THE IMPACT OF CHROMOSPHERIC ACTIVITY ON OBSERVED INITIAL MASS FUNCTIONS

Using recently established empirical calibrations for the impact of chromospheric activity on the radii, effective temperatures, and estimated masses of active low-mass stars and brown dwarfs, we reassess the shape of the initial mass function (IMF) across the stellar/substellar boundary in the Upper Sco star-forming region (age 5-10 Myr). We adjust the observed effective temperatures to warmer values using the observed strength of the chromospheric H

How do stars gain their mass? : A JCMT/SCUBA-2 Transient Survey of Protostars in Nearby Star Forming Regions

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The disk around the brown dwarf KPNO Tau 3

emission, and redetermine the estimated masses of objects using pre--main-sequence evolutionary tracks in the H-R diagram. The effect of the activity-adjusted temperatures is to shift the objects to higher masses by 3-100%. While the slope of the resulting IMF at substellar masses is not strongly changed, the peak of the IMF does shift from ~0.06 to ~0.11 Msun. Moreover, for objects with masses <~0.2 Msun, the ratio of brown dwarfs to stars changes from ~80% to ~33%. These results suggest that activity corrections are essential for studies of the substellar mass function, if the masses are estimated from spectral types or from effective temperatures.

The JCMT Transient Survey: Stochastic and Secular Variability of Protostars and Disks In the Submillimeter Region Observed over 18 Months

Most protostars have luminosities that are fainter than expected from steady accretion over the protostellar lifetime. The solution to this problem may lie in episodic mass accretion—prolonged periods of very low accretion punctuated by short bursts of rapid accretion. However, the timescale and amplitude for variability at the protostellar phase is almost entirely unconstrained. In A James Clerk Maxwell Telescope/SCUBA-2 Transient Survey of Protostars in Nearby Star-forming Regions, we are monitoring monthly with SCUBA-2 the submillimeter emission in eight fields within nearby (<500 pc) star-forming regions to measure the accretion variability of protostars. The total survey area of ~1.6 deg^2 includes ~105 peaks with peaks brighter than 0.5 Jy/beam (43 associated with embedded protostars or disks) and 237 peaks of 0.125–0.5 Jy/beam (50 with embedded protostars or disks). Each field has enough bright peaks for flux calibration relative to other peaks in the same field, which improves upon the nominal flux calibration uncertainties of submillimeter observations to reach a precision of ~2%–3% rms, and also provides quantified confidence in any measured variability. The timescales and amplitudes of any submillimeter variation will then be converted into variations in accretion rate and subsequently used to infer the physical causes of the variability. This survey is the first dedicated survey for submillimeter variability and complements other transient surveys at optical and near-IR wavelengths, which are not sensitive to accretion variability of deeply embedded protostars.

The JCMT Transient Survey : identifying submillimeter continuum variability over several year timescales using archival JCMT Gould Belt Survey observations

We present submillimeter observations of the young brown dwarfs KPNO Tau 1, KPNO Tau 3, and KPNO Tau 6 at 450 micron and 850 micron taken with the Submillimeter Common-User Bolometer Array on the James Clerke Maxwell Telescope. KPNO Tau 3 and KPNO Tau 6 have been previously identified as Class II objects hosting accretion disks, whereas KPNO Tau 1 has been identified as a Class III object and shows no evidence of circumsubstellar material. Our 3 sigma detection of cold dust around KPNO Tau 3 implies a total disk mass of (4.0 +/- 1.1) x 10^{-4} Msolar (assuming a gas to dust ratio of 100:1). We place tight constraints on any disks around KPNO Tau 1 or KPNO Tau 6 of <2.1 x 10^{-4} Msolar and <2.7 x 10^{-4} Msolar, respectively. Modeling the spectral energy distribution of KPNO Tau 3 and its disk suggests the disk properties (geometry, dust mass, and grain size distribution) are consistent with observations of other brown dwarf disks and low-mass T-Tauri stars. In particular, the disk-to-host mass ratio for KPNO Tau 3 is congruent with the scenario that at least some brown dwarfs form via the same mechanism as low-mass stars.

A Universal Spin–Mass Relation for Brown Dwarfs and Planets

We analyze results from the first eighteen months of monthly sub-mm monitoring of eight star-forming regions in the JCMT Transient Survey. In our search for stochastic variability in 1643 bright peaks, only the previously identified source, EC53, shows behavior well above the expected measurement uncertainty. Another four sources, two disks and two protostars, show moderately-enhanced standard deviations in brightness, as expected for stochastic variables. For the two protostars, this apparent variability is the result of single epochs that are much brighter than the mean. In our search for secular brightness variations that are linear in time, we measure the fractional brightness change per year for 150 bright peaks, fifty of which are protostellar. The ensemble distribution of slopes is well fit by a normal distribution with sigma ~ 0.023. Most sources are not rapidly brightening or fading in the sub-mm. Comparison against time-randomized realizations shows that the width of the distribution is dominated by the uncertainty in the individual brightness measurements of the sources. A toy model for secular variability reveals that an underlying Gaussian distribution of linear fractional brightness change sigma = 0.005 would be unobservable in the present sample, whereas an underlying distribution with sigma = 0.02 is ruled out. Five protostellar sources, 10% of the protostellar sample, are found to have robust secular measures deviating from a constant flux. The sensitivity to secular brightness variations will improve significantly with a larger time sample, with a factor of two improvement expected by the conclusion of our 36-month survey.

Activity and rotation of low mass stars in young open clusters

Investigating variability at the earliest stages of low-mass star formation is fundamental in understanding how a protostar assembles mass. While many simulations of protostellar disks predict non-steady accretion onto protostars, deeper investigation requires robust observational constraints on the frequency and amplitude of variability events characterised across the observable SED. In this study, we develop methods to robustly analyse repeated observations of an area of the sky for submillimetre variability in order to determine constraints on the magnitude and frequency of deeply embedded protostars. We compare \mbox{850

Very Low-mass Stars and Brown Dwarfs in Upper Scorpius Using Gaia DR1: Mass Function, Disks, and Kinematics

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