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Papers by Keaton Bell

The Astrophysical Journal, 2013

We report the discovery of the second and third pulsating extremely low mass white dwarfs (WDs), ... more We report the discovery of the second and third pulsating extremely low mass white dwarfs (WDs), SDSS J111215.82+111745.0 (hereafter J1112) and SDSS J151826.68+065813.2 (hereafter J1518). Both have masses < 0.25 M ⊙ and effective temperatures below 10, 000 K, establishing these putatively He-core WDs as a cooler class of pulsating hydrogen-atmosphere WDs (DAVs, or ZZ Ceti stars). The short-period pulsations evidenced in the light curve of J1112 may also represent the first observation of acoustic (p-mode) pulsations in any WD, which provide an exciting opportunity to probe this WD in a complimentary way compared to the long-period g-modes also present. J1112 is a T eff = 9590 ± 140 K and log g = 6.36 ± 0.06 WD. The star displays sinusoidal variability at five distinct periodicities between 1792 − 2855 s. In this star we also see short-period variability, strongest at 134.3 s, well short of expected g-modes for such a low-mass WD. The other new pulsating WD, J1518, is a T eff = 9900±140 K and log g = 6.80±0.05 WD. The light curve of J1518 is highly non-sinusoidal, with at least seven significant periods between 1335 − 3848 s. Consistent with the expectation that ELM WDs must be formed in binaries, these two new pulsating He-core WDs, in addition to the prototype SDSS J184037.78+642312.3, have close companions. However, the observed variability is inconsistent with tidally induced pulsations and is so far best explained by the same hydrogen partial-ionization driving mechanism at work in classic C/O-core ZZ Ceti stars.

The Astronomical Journal, 2011

We analyze the magnetic activity of 59,318 M dwarfs from the Sloan Digital Sky Survey (SDSS) Data... more We analyze the magnetic activity of 59,318 M dwarfs from the Sloan Digital Sky Survey (SDSS) Data Release 7. This analysis explores the spatial distribution of M dwarf activity as a function of both vertical distance from the Galactic plane (Z) and planar distance from the Galactic center (R). We confirm the established trends of decreasing magnetic activity (as measured by Hα emission) with increasing distance from the mid-plane of the disk and find evidence for a trend in Galactocentric radius. We measure a non-zero radial gradient in the activity fraction in our analysis of stars with spectral types dM3 and dM4. The activity fraction increases with R and can be explained by a decreasing mean stellar age with increasing distance from the Galactic center.

Publications of the Astronomical Society of the Pacific, 2012

We use ∼12,000 spectra of ∼3,500 magnetically active M0-M9 dwarfs from the Sloan Digital Sky Surv... more We use ∼12,000 spectra of ∼3,500 magnetically active M0-M9 dwarfs from the Sloan Digital Sky Survey taken at 10-15 minute intervals, together with ∼300 spectra of ∼60 M0-M8 stars obtained hourly with the Hydra multi-object spectrometer, to probe Hα variability on timescales of minutes to weeks. With multiple observations for every star examined, we are able to characterize fluctuations in Hα emission as a function of activity strength and spectral type. Stars with greater magnetic activity (as quantified by L Hα /L bol ) are found to be less variable at all spectral types. We attribute this result to the stronger level of persistent emission in the high activity stars, requiring a larger heating event in order to produce measurable variability. We also construct Hα structure functions to constrain the timescale of variability. The more active objects with lower variability exhibit a characteristic timescale longer than an hour, likely due to larger, longer lasting heating events, while the less active objects with higher variability have a characteristic timescale shorter than 15 minutes.

The Astrophysical Journal, 2013

We report the discovery of the second and third pulsating extremely low mass white dwarfs (WDs), ... more We report the discovery of the second and third pulsating extremely low mass white dwarfs (WDs), SDSS J111215.82+111745.0 (hereafter J1112) and SDSS J151826.68+065813.2 (hereafter J1518). Both have masses < 0.25 M ⊙ and effective temperatures below 10, 000 K, establishing these putatively He-core WDs as a cooler class of pulsating hydrogen-atmosphere WDs (DAVs, or ZZ Ceti stars). The short-period pulsations evidenced in the light curve of J1112 may also represent the first observation of acoustic (p-mode) pulsations in any WD, which provide an exciting opportunity to probe this WD in a complimentary way compared to the long-period g-modes also present. J1112 is a T eff = 9590 ± 140 K and log g = 6.36 ± 0.06 WD. The star displays sinusoidal variability at five distinct periodicities between 1792 − 2855 s. In this star we also see short-period variability, strongest at 134.3 s, well short of expected g-modes for such a low-mass WD. The other new pulsating WD, J1518, is a T eff = 9900±140 K and log g = 6.80±0.05 WD. The light curve of J1518 is highly non-sinusoidal, with at least seven significant periods between 1335 − 3848 s. Consistent with the expectation that ELM WDs must be formed in binaries, these two new pulsating He-core WDs, in addition to the prototype SDSS J184037.78+642312.3, have close companions. However, the observed variability is inconsistent with tidally induced pulsations and is so far best explained by the same hydrogen partial-ionization driving mechanism at work in classic C/O-core ZZ Ceti stars.

The Astronomical Journal, 2011

We analyze the magnetic activity of 59,318 M dwarfs from the Sloan Digital Sky Survey (SDSS) Data... more We analyze the magnetic activity of 59,318 M dwarfs from the Sloan Digital Sky Survey (SDSS) Data Release 7. This analysis explores the spatial distribution of M dwarf activity as a function of both vertical distance from the Galactic plane (Z) and planar distance from the Galactic center (R). We confirm the established trends of decreasing magnetic activity (as measured by Hα emission) with increasing distance from the mid-plane of the disk and find evidence for a trend in Galactocentric radius. We measure a non-zero radial gradient in the activity fraction in our analysis of stars with spectral types dM3 and dM4. The activity fraction increases with R and can be explained by a decreasing mean stellar age with increasing distance from the Galactic center.

Publications of the Astronomical Society of the Pacific, 2012

We use ∼12,000 spectra of ∼3,500 magnetically active M0-M9 dwarfs from the Sloan Digital Sky Surv... more We use ∼12,000 spectra of ∼3,500 magnetically active M0-M9 dwarfs from the Sloan Digital Sky Survey taken at 10-15 minute intervals, together with ∼300 spectra of ∼60 M0-M8 stars obtained hourly with the Hydra multi-object spectrometer, to probe Hα variability on timescales of minutes to weeks. With multiple observations for every star examined, we are able to characterize fluctuations in Hα emission as a function of activity strength and spectral type. Stars with greater magnetic activity (as quantified by L Hα /L bol ) are found to be less variable at all spectral types. We attribute this result to the stronger level of persistent emission in the high activity stars, requiring a larger heating event in order to produce measurable variability. We also construct Hα structure functions to constrain the timescale of variability. The more active objects with lower variability exhibit a characteristic timescale longer than an hour, likely due to larger, longer lasting heating events, while the less active objects with higher variability have a characteristic timescale shorter than 15 minutes.