A statistical method for measuring the Galactic potential and testing gravity with cold tidal streams

We introduce the Minimum Entropy Method, a simple statistical technique for constraining the Milky Way gravitational potential and simultaneously testing different gravity theories directly from 6D phase-space surveys and without adopting dynamical models. We demonstrate that orbital energy distributions that are separable (i.e. independent of position) have an associated entropy that increases under wrong assumptions about the gravitational potential and/or gravity theory. Of known objects, `cold' tidal streams from low-mass progenitors follow orbital distributions that most nearly satisfy the condition of separability. Although the orbits of tidally stripped stars are perturbed by the progenitor's self-gravity, systematic variations of the energy distribution can be quantified in terms of the cross-entropy of individual tails, giving further sensitivity to theoretical biases in the host potential. The feasibility of using the Minimum Entropy Method to test a wide range of gravity theories is illustrated by evolving restricted N-body models in a Newtonian potential and examining the changes in entropy introduced by Dirac, MONDian and f(R) gravity modifications.Comment: Accepted for publication in ApJ. 11 pages 6 figure

Clean Kinematic Samples in Dwarf Spheroidals: An Algorithm for Evaluating Membership and Estimating Distribution Parameters When Contamination is Present

(abridged) We develop an algorithm for estimating parameters of a distribution sampled with contamination, employing a statistical technique known as ``expectation maximization'' (EM). Given models for both member and contaminant populations, the EM algorithm iteratively evaluates the membership probability of each discrete data point, then uses those probabilities to update parameter estimates for member and contaminant distributions. The EM approach has wide applicability to the analysis of astronomical data. Here we tailor an EM algorithm to operate on spectroscopic samples obtained with the Michigan-MIKE Fiber System (MMFS) as part of our Magellan survey of stellar radial velocities in nearby dwarf spheroidal (dSph) galaxies. These samples are presented in a companion paper and contain discrete measurements of line-of-sight velocity, projected position, and Mg index for ~1000 - 2500 stars per dSph, including some fraction of contamination by foreground Milky Way stars. The EM algorithm quantifies both dSph and contaminant distributions, returning maximum-likelihood estimates of the means and variances, as well as the probability that each star is a dSph member. Applied to our MMFS data, the EM algorithm identifies more than 5000 probable dSph members. We test the performance of the EM algorithm on simulated data sets that represent a range of sample size, level of contamination, and amount of overlap between dSph and contaminant velocity distributions. The simulations establish that for samples ranging from large (N ~3000) to small (N~30), the EM algorithm distinguishes members from contaminants and returns accurate parameter estimates much more reliably than conventional methods of contaminant removal (e.g., sigma clipping).Comment: Accepted for publication in The Astronomical Journal. Download pdf with full-resolution figures from http://www.ast.cam.ac.uk/~walker/dsph_em.pd

Systemic Proper Motions of Milky Way Satellites from Stellar Redshifts: the Carina, Fornax, Sculptor and Sextans Dwarf Spheroidals

The transverse motions of nearby dwarf spheroidal (dSph) galaxies contribute line-of-sight components that increase with angular distance from the dSph centers, inducing detectable gradients in stellar redshift. In the absence of an intrinsic velocity gradient (e.g., due to rotation or streaming), an observed gradient in the heliocentric rest frame (HRF) relates simply to a dSph's systemic proper motion (PM). Kinematic samples for the Milky Way's brightest dSph satellites are now sufficiently large that we can use stellar redshifts to constrain systemic PMs independently of astrometric data. Data from our Michigan/MIKE Fiber System (MMFS) Survey reveal significant HRF velocity gradients in Carina, Fornax and Sculptor, and no significant gradient in Sextans. Assuming there are no intrinsic gradients, the data provide a relatively tight constraint on the PM of Fornax, (mu_{alpha}^{HRF},mu_{delta}^{HRF})=(+48 +/- 15,-25 +/- 14) mas/century, that agrees with published HST astrometric measurements. Smaller data sets yield weaker constraints in the remaining galaxies, but our Carina measurement, (mu_{alpha}^{HRF},mu_{delta}^{HRF})=(+25 +/- 36,+16 +/- 43) mas/century, agrees with the published astrometric value. The disagreement of our Sculptor measurement, (mu_{alpha}^{HRF},mu_{delta}^{HRF})= (-40 +/- 29, -69 +/- 47) mas/century, with astrometric measurements is expected if Sculptor has a rotational component as reported by Battaglia et al. (2008). For Sextans, which at present lacks an astrometric measurement, we measure (mu_{alpha}^{HRF},mu_{delta}^{HRF})=(-26 +/- 41, +10 +/- 44) mas/century.Comment: Accepted for Publication by ApJ Letter

Reducing the Stigma of Mental Illness Among Medical Students

Abstract: The American Osteopathic Association House of Delegates Resolution 205 recommends “increased awareness of depression amongst U.S. Medical students” due to the increasing body of research describing the rise of depression, burn-out and suicide ideation among medical students. There is consequently a need to understand mental health issues as a component of professional development. Hypothesis: A student-led symposium addressing mental and emotional health topics relevant to medical students would reduce the stigma associated with mental illness. Materials and Methods: A 2-hour student-run “Patient Perspective” was held during the second neuroscience block at an osteopathic medical school in the northeastern United States. One week before the program, a student-developed online Wellness Survey measured prevalence of mental illness, common feelings during medical school, coping mechanisms used for stress, and use of mental health resources. Immediately before and after the program, students were asked to report their familiarity with mental illness and their feelings regarding a vignette about a mentally ill woman using “Mental Illness Among Us” pre and post surveys provided by the University of California San Francisco School of Medicine and adapted for the event. During the program, data from the online survey were shared, student organizers discussed emotional wellness and positive coping mechanisms in the context of the profession, and student panelists shared their experiences with mental health issues. A faculty psychiatrist spoke about mental health resources, and attendees received pamphlets listing these resources. The event concluded with student-led breakout sessions at which stress during medical school and strategies for promoting positive coping mechanisms were discussed, followed by the post survey. Results: 113 students completed the pre survey, 89 of whom completed the post survey. For these 89, differences between post and pre responses were universally in the direction of increasing acceptance and decreasing stigma of those with mental illness; all differences were statistically significant. The largest shift regarded students’ reluctance to disclose their own theoretical mental illness to colleagues. Conclusion: Incorporating an emotional health symposium into medical students’ training may increase understanding and acceptance of those who may have mental illness and reduce stigma associated with mental illness.https://digitalcommons.pcom.edu/posters/1002/thumbnail.jp

Reducing the stigma of mental illness among medical students

Background: The American Osteopathic Association House of Delegates Resolution 205 recommends “increased awareness of depression amongst U.S. medical students” due to the increasing body of research describing the rise of depression, burn-out and suicide ideation among medical students. There is consequently a need to understand mental health issues as a component of professional development. Hypothesis: A student-led symposium addressing mental and emotional health topics relevant to medical students will reduce the stigma associated with mental illness. Materials and Methods: A 2-hour student-run “Patient Perspective” session was held during the second year neuroscience block in the PCOM DO program. One week before the program, a student-developed, online Wellness Survey measured prevalence of mental illness, common feelings during medical school, coping mechanisms used for stress, and use of mental health resources. Immediately before and after the program, students were asked to report their familiarity with mental illness and their feelings regarding a vignette about a mentally ill woman. Pre- and post-activity surveys were provided by the University of California San Francisco School of Medicine and adapted for the event. During the program, data from the online survey were shared, student organizers discussed emotional wellness and positive coping mechanisms in the context of the profession, and student panelists shared their experiences with mental health issues. A faculty psychiatrist spoke about mental health resources, and attendees received pamphlets listing these resources. The event concluded with student-led breakout sessions in which stress during medical school and strategies for promoting positive coping mechanisms were discussed, followed by administration of the post-activity survey. Results: 113 students completed the pre-activity survey; 89 completed the post-activity survey. For these 89, differences between responses were universally in the direction of increasing acceptance and decreasing stigma of those with mental illness; all differences were statistically significant. The largest shift regarded students’ reluctance to disclose their own theoretical mental illness to colleagues. Conclusion: Incorporating an emotional health symposium into medical students’ training may increase understanding and acceptance of those who may have mental illness and reduce stigma associated with mental illness

Physical Conditions of Accreting Gas in T Tauri Star Systems

We present results from a low resolution (R~300) near-infrared spectroscopic variability survey of actively accreting T Tauri stars (TTS) in the Taurus-Auriga star forming region. Paschen and Brackett series H I recombination lines were detected in 73 spectra of 15 classical T Tauri systems. The values of the Pan/PaB, Brn/BrG, and BrG/Pan H I line ratios for all observations exhibit a scatter of < 20% about the weighted mean, not only from source to source, but also for epoch-to-epoch variations in the same source. A representative or `global' value was determined for each ratio in both the Paschen and Brackett series as well as the BrG/Pan line ratios. A comparison of observed line ratio values was made to those predicted by the temperature and electron density dependent models of Case B hydrogen recombination line theory. The measured line ratios are statistically well-fit by a tightly constrained range of temperatures (T < 2000 K) and electron densities 1e9 < n_e < 1e10 cm^-3. A comparison of the observed line ratio values to the values predicted by the optically thick and thin local thermodynamic equilibrium cases rules out these conditions for the emitting H I gas. Therefore, the emission is consistent with having an origin in a non-LTE recombining gas. While the range of electron densities is consistent with the gas densities predicted by existing magnetospheric accretion models, the temperature range constrained by the Case B comparison is considerably lower than that expected for accreting gas. The cooler gas temperatures will require a non-thermal excitation process (e.g., coronal/accretion-related X-rays and UV photons) to power the observed line emission.Comment: 12 pages, emulateapj format, Accepted for publication in Ap