On the impact of Helium abundance on the Cepheid Period-Luminosity and Wesenheit relations and the Distance Ladder

This work analyses the effect of the Helium content on synthetic Period-Luminosity Relations (PLRs) and Period-Wesenheit Relations (PWRs) of Cepheids and the systematic uncertainties on the derived distances that a hidden population of He-enhanced Cepheids may generate. We use new stellar and pulsation models to build a homogeneous and consistent framework to derive the Cepheid features. The Cepheid populations expected in synthetic color-magnitude diagrams of young stellar systems (from 20 Myr to 250 Myr) are computed in several photometric bands for Y = 0.25 and Y = 0.35, at a fixed metallicity (Z = 0.008). The PLRs appear to be very similar in the two cases, with negligible effects (few %) on distances, while PWRs differ somewhat, with systematic uncertainties in deriving distances as high as about 7% at log P < 1.5. Statistical effects due to the number of variables used to determine the relations contribute to a distance systematic error of the order of few percent, with values decreasing from optical to near-infrared bands. The empirical PWRs derived from multi-wavelength datasets for the Large Magellanic Cloud (LMC) is in a very good agreement with our theoretical PWRs obtained with a standard He content, supporting the evidence that LMC Cepheids do not show any He effect

Multipopulation aftereffects on the color-magnitude diagram and Cepheid variables of young stellar systems

Context: The evidence of a multipopulation scenario in Galactic globular clusters raises several questions about the formation and evolution of the two (or more) generations of stars. These populations show differences in their age and chemical composition. These differences are found in old- and intermediate- age stellar clusters in the Local Group. The observations of young stellar systems are expected to present footprints of multiple stellar populations. Aims: This theoretical work intends to be a specific step in exploring the space of the observational indicators of multipopulations, without covering all the combinations of parameters that may contribute to the formation of multiple generations of stars in a cluster or in galaxy. The goal is to shed light on the possible observational features expected by core He-burning stars that belong to two stellar populations with different original He content and ages. Methods: The tool adopted was the stellar population synthesis. We used new stellar and pulsation models to construct a homogeneous and consistent framework. Synthetic color-magnitude diagrams (CMDs) of young- and intermediate-age stellar systems (from 20 Myr up to 1 Gyr) were computed in several photometric bands to derive possible indicators of double populations both in the observed CMDs and in the pulsation properties of the Cepheids. Results: We predict that the morphology of the red/blue clump in VIK bands can be used to photometrically indicate the two stellar populations in a rich assembly of stars if there is a significant difference in their original He content. Moreover, the period distribution of the Cepheids appears to be widely affected by the coeval multiple generations of stars within stellar systems. We show that the Wesenheit relations may be affected by the helium content of the Cepheids.Comment: in press on A&

Academic Capitalism And Historically Black Colleges And Universities: Institutional Conflict

The relevance of historically Black colleges and universities (HBCUs) in the current educational climate remain a critical issue. A mixed-methods case study was used to examine the conflicting concerns among faculty at a private HBCU in northeast Texas that has in recent years faced financial distress, declining enrollment, and administrative leadership turnover. The research design incorporated a two-step, critical race process that examined ‘faculty concerns’ on two hypothesized dimensions: academic capitalism versus academic autonomy. Relying on the meta-theory of institutional logics, the study examined the embedded racial structure of market-based metrics associated with HBCU faculty caught in a wave of ‘academic capitalism’ and the consequent paradox of trying to maintain their traditional role as scholastic gatekeepers. The findings suggest two institutional logics—neoliberalism at the administrative level and faculty autonomy at the academic level—were in conflict. It is recommend that HBCU stakeholders recognize the differences in institutional logics affecting faculty perceptions to mitigate the ongoing crises associated with administrators, finances, accreditation, and academic standards. Limitations and future directions for research are discussed

Competing Institutional Logics And Teaching Effectiveness In Traditional And Online University Classrooms

Relying on an institutional logics framework, we use a case study method to investigate competing inter-institutional logics effecting U.S. postsecondary teaching effectiveness ratings in traditional and online courses at a midsize Texas public university. Prior research attributes differences to instructor and student attitudes, performative characteristics, and motivation but few studies have examined evaluation outcomes in light of competing logics that contextualize administrators, faculty, and students’ practices in the qualitatively different classroom settings. Using a multilevel latent factor model, we correlated variances in students’ assessments on key institutional criteria and compare differences in students’ teaching effectiveness ratings between the two settings. We theorized that different neoliberal dispositions emerge from competing institutional logics framing actors\u27 normative assumptions in traditional and online classrooms. The findings indicate that instructors’ significantly lower evaluations in online classes were linked to competing institutional logics affecting actors’ cognitions and practices. Noteworthy was students’ assessments were not gender biased from an institutional logics perspective in either instructional field

Detection of Radial Surface Brightness Fluctuation and Color Gradients in elliptical galaxies with ACS

We study surface brightness fluctuations (SBF) in a sample of 8 elliptical galaxies using Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) data drawn from the Hubble Space Telescope (HST) archive. SBF magnitudes in the F814W bandpass, and galaxy colors from F814W, F435W, and F606W images -- when available -- are presented. Galaxy surface brightness profiles are determined as well. We present the first SBF--broadband color calibration for the ACS/WFC F814W bandpass, and (relative) distance moduli estimates for 7 of our galaxies. We detect and study in detail the SBF variations within individual galaxies as a probe of possible changes in the underlying stellar populations. Inspecting both the SBF and color gradients in comparison to model predictions, we argue that SBF, and SBF-gradients, can in principle be used for unraveling the different evolutionary paths taken by galaxies, though a more comprehensive study of this issue would be required. We confirm that the radial variation of galaxy stellar population properties should be mainly connected to the presence of radial chemical abundance gradients, with the outer galaxy regions being more metal poor than the inner ones.Comment: 47 pages, 13 figures, ApJ, accepte

Student perceptions and instructional evaluations: A multivariate analysis of online and face-to-face classroom settings

Authors copy of an article originally printed in Education and Information TechnologiesThis study examined students’ evaluations of faculty performance in traditional and online classes. The study design builds upon prior research that addressed socially relevant factors such as classroom environments, students’ learning goals, expected, and received grades, and more importantly, students’ ratings of instructors’ performance. The sample consists of data from a population of humanities and social sciences faculty from a medium-sized southwest undergraduate university who taught both online and traditional classes during the semester periods Fall 2010 to Spring 2012. In a traditional setting, the evaluation factors (develops rapport with students, stimulates students, challenges student learning, provides timely feedback, and teaches fundamentals), and the external factors—(course level taught and gender)—were found to significantly contribute to faculty summary scores. In an online class, students consistently rank female instructors better. However, the evaluation criteria—develops student rapport, stimulates students, provides timely feedback, and teaches fundamentals (though not ‘challenges and involves students in their learning’)—mirrored the same affects observed in the traditional classroom evaluations. The finding that “teaches fundamentals” received the largest standardized beta-coefficient in both classrooms further confirms earlier research that university students perceive course mastery as a major indicator of instructor performance regardless of gender or rank. However, the results indicate that students’ perceptions are different when attending a traditional versus online classroom setting. This infers that synchronous and asynchronous settings require different teaching styles and different evaluation criteria.Sociolog

The He abundance in NGC 1850 A and B: are we observing the early stage of formation of multiple populations in a stellar cluster?

We present the result of a sample of B-stars in the Large Magellanic Cloud young double stellar cluster NGC 1850 A and NGC 1850 B, observed with the integral-field spectrograph at the Very Large Telescope, the Multi Unit Spectroscopic Explorer. We compare the observed equivalent widths (EWs) of four He lines (4922 $\mathring{\mathrm A}$, 5015 $\mathring{\mathrm A}$, 6678 $\mathring{\mathrm A}$, and 7065 $\mathring{\mathrm A}$) with the ones determined from synthetic spectra computed with different He mass fraction (Y=0.25, 0.27, 0.30 and 0.35) with the code SYNSPEC, that takes into account the non-LTE effect. From this comparison, we determined the He mass fraction of the B stars, finding a not homogeneous distribution. The stars can be divided in three groups, He-weak (Y $\lt$ 0.24) and the He-normal (0.24 $\leqslant$ Y $\leqslant$ 0.26) belonging to the MS of NGC 1850 A, and the He-rich stars (0.33 $\leqslant$ Y $\leqslant$ 0.38) situated in the MS associated to NGC 1850 B. We have analyzed the stellar rotation as possible responsible of the anomalous features of the He lines in the He-rich stars. We provide a simple analysis of the differences between the observed EWs and the ones obtained from the theoretical models with different rotation velocity (V$\sin{i}$ = 0 and 250 Km/s). The resolution of the MUSE spectra do not allow to get a conclusive result, however our analysis support the He-enhanced hypothesis.Comment: Accepted for publication by MNRAS, 10 pages, 8 figure

The luminosity function of the Large Magellanic Cloud globular cluster NGC 1866

We present {\it Hubble Space Telescope} {\it V,I} photometry of the central region of the LMC cluster NGC 1866, reaching magnitudes as faint as V=27 mag. We find evidence that the cluster luminosity function shows a strong dependence on the distance from the cluster center, with a clear deficiency of low luminosity stars in the inner region. We discuss a {\it global} cluster luminosity function as obtained from stars in all the investigated region, which appears in impressive agreement with the prediction from a Salpeter mass distribution. We also revisit the use of NGC 1866 as a probe for determining the efficiency of core overshooting, and conclude that a definitive answer to this question is not possible from this cluster.Comment: AJ accepted, 16 pages, 19 figures, uses aastex.cl

Early metal enrichment in high-redshift quasars

Quasars are powerful systems whose spectrum is rich of metal features that allow us to investigate the chemical evolution of galaxies at very high redshift, even close to the reionization epoch. I review the main observational constraints on the metallicity of quasars host galaxies at high redshift and discuss the implications and issues for models of galaxy evolution in the early universe.Comment: 8 pages, invited review at the workshop "Probing Stellar Populations out to the Distant Universe