The Star Formation History of the Large Magellanic Cloud

Using WFPC2 aboard the Hubble Space Telescope, we have created deep color-magnitude diagrams in the V and I passbands for approximately 100,000 stars in a field at the center of the LMC bar and another in the disk. The main--sequence luminosity functions (LFs) from 19 mag < V < 23.5 mag, the red clump and horizontal branch morphologies, and the differential Hess diagram of the two fields all strongly imply that the disk and bar have significantly different star-formation histories (SFHs). The disk's SFH has been relatively smooth and continuous over the last 15 Gyr while the bar's SFH was dominated by star formation episodes at intermediate ages. Comparison of the LF against predictions based on Padova theoretical stellar evolution models and an assumed age-metallicity relationship allows us to identify the dominant stellar populations in the bar with episodes of star formation that occurred from 4 to 6 and 1 to 2 Gyr ago. These events accounted, respectively, for approximately 25% and 15% of its stellar mass. The disk field may share a mild enhancement in SF for the younger episode, and thus we identify the 4 to 6 Gyr episode with the formation of the LMC bar.Comment: 14 pages, 5 figures, Latex, also available at http://www.ps.uci.edu/physics/smeckerhane.html. Accepted for publication in Ap

Mass Segregation in the Globular Cluster Palomar 5 and its Tidal Tails

We present the stellar main sequence luminosity function (LF) of the disrupted, low-mass, low-concentration globular cluster Palomar 5 and its well-defined tidal tails, which emanate from the cluster as a result of its tidal interaction with the Milky Way. The results of our deep (B ~ 24.5) wide-field photometry unequivocally indicate that preferentially fainter stars were removed from the cluster so that the LF of the cluster's main body exhibits a significant degree of flattening compared to other globular clusters. There is clear evidence of mass segregation, which is reflected in a radial variation of the LFs. The LF of the tidal tails is distinctly enhanced with faint, low-mass stars. Pal 5 exhibits a binary main sequence, and we estimate a photometric binary frequency of roughly 10%. Also the binaries show evidence of mass segregation with more massive binary systems being more strongly concentrated toward the cluster center.Comment: 14 pages, 12 figures, accepted for publication in the Astronomical Journa

Discovery of a Gas-Rich Companion to the Extremely Metal-Poor Galaxy DDO 68

We present HI spectral-line imaging of the extremely metal-poor galaxy DDO 68. This system has a nebular oxygen abundance of only 3% Z$_{\odot}$, making it one of the most metal-deficient galaxies known in the local volume. Surprisingly, DDO 68 is a relatively massive and luminous galaxy for its metal content, making it a significant outlier in the mass-metallicity and luminosity-metallicity relationships. The origin of such a low oxygen abundance in DDO 68 presents a challenge for models of the chemical evolution of galaxies. One possible solution to this problem is the infall of pristine neutral gas, potentially initiated during a gravitational interaction. Using archival HI spectral-line imaging obtained with the Karl G. Jansky Very Large Array, we have discovered a previously unknown companion of DDO 68. This low-mass (M$_{\rm HI}$ $=$ 2.8$\times$10$^{7}$ M$_{\odot}$), recently star-forming (SFR$_{\rm FUV}$ $=$ 1.4$\times$10$^{-3}$ M$_{\odot}$ yr$^{-1}$, SFR$_{\rm H\alpha}$ $<$ 7$\times$10$^{-5}$ M$_{\odot}$ yr$^{-1}$) companion has the same systemic velocity as DDO 68 (V$_{\rm sys}$ $=$ 506 km s$^{-1}$; D $=$ 12.74$\pm$0.27 Mpc) and is located at a projected distance of 42 kpc. New HI maps obtained with the 100m Robert C. Byrd Green Bank Telescope provide evidence that DDO 68 and this companion are gravitationally interacting at the present time. Low surface brightness HI gas forms a bridge between these objects.Comment: Accepted for publication in the Astrophysical Journal Letter

Observations and Implications of the Star Formation History of the LMC

We present derivations of star formation histories based on color-magnitude diagrams of three fields in the LMC from HST/WFPC2 observations. A significant component of stars older than 4 Gyr is required to match the observed color-magnitude diagrams. Models with a dispersion-free age-metallicity relation are unable to reproduce the width of the observed main sequence; models with a range of metallicity at a given age provide a much better fit. Such models allow us to construct complete ``population boxes'' for the LMC based entirely on color-magnitude diagrams; remarkably, these qualitatively reproduce the age-metallicity relation observed in LMC clusters. We discuss some of the uncertainties in deriving star formation histories. We find, independently of the models, that the LMC bar field has a larger relative component of older stars than the outer fields. The main implications suggested by this study are: 1) the star formation history of field stars appears to differ from the age distribution of clusters, 2) there is no obvious evidence for bursty star formation, but our ability to measure bursts shorter in duration than $\sim$ 25% of any given age is limited by the statistics of the observed number of stars, 3) there may be some correlation of the star formation rate with the last close passage of the LMC/SMC/Milky Way, but there is no dramatic effect, and 4) the derived star formation history is probably consistent with observed abundances, based on recent chemical evolution models.Comment: Accepted by AJ, 36 pages including 12 figure

Stellar Populations in Three Outer Fields of the LMC

We present HST photometry for three fields in the outer disk of the LMC extending approximately four magnitudes below the faintest main sequence turnoff. We cannot detect any strongly significant differences in the stellar populations of the three fields based on the morphologies of the color-magnitude diagrams, the luminosity functions, and the relative numbers of stars in different evolutionary stages. Our observations therefore suggest similar star formation histories in these regions, although some variations are certainly allowed. The fields are located in two regions of the LMC: one is in the north-east field and two are located in the north-west. Under the assumption of a common star formation history, we combine the three fields with ground-based data at the same location as one of the fields to improve statistics for the brightest stars. We compare this stellar population with those predicted from several simple star formation histories suggested in the literature, using a combination of the R-method of Bertelli et al (1992) and comparisons with the observed luminosity function. The only model which we consider that is not rejected by the observations is one in which the star formation rate is roughly constant for most of the LMC's history and then increases by a factor of three about 2 Gyr ago. Such a model has roughly equal numbers of stars older and younger than 4 Gyr, and thus is not dominated by young stars. This star formation history, combined with a closed box chemical evolution model, is consistent with observations that the metallicity of the LMC has doubled in the past 2 Gyr.Comment: 30 pages, includes 10 postscript figures. Figure 1 avaiable at ftp://charon.nmsu.edu/pub/mgeha/LMC. Accepted for publication in Astronomical Journa

Genome-Wide Association Studies of Coffee Intake in UK/US Participants of European Ancestry Uncover Gene-Cohort Influences

Coffee is one of the most widely consumed beverages. We performed a genome-wide association study (GWAS) of coffee intake in US-based 23andMe participants (N =130,153) and identified 7 significant loci, with many replicating in three multi-ancestral cohorts. We examined genetic correlations and performed a phenome-wide association study across thousands of biomarkers and health and lifestyle traits, then compared our results to the largest available GWAS of coffee intake from UK Biobank (UKB; N =334,659). The results of these two GWAS were highly discrepant. We observed positive genetic correlations between coffee intake and psychiatric illnesses, pain, and gastrointestinal traits in 23andMe that were absent or negative in UKB. Genetic correlations with cognition were negative in 23andMe but positive in UKB. The only consistent observations were positive genetic correlations with substance use and obesity. Our study shows that GWAS in different cohorts could capture cultural differences in the relationship between behavior and genetics

Measuring Health Related Quality of Life for Dengue Patients in Iquitos, Peru

Previous studies measuring the health-related quality of life (HRQoL) of individuals with dengue focused on treatment seeking populations. However, the vast majority of global dengue cases are unlikely to be detected by health systems. Representative measurements of HRQoL should therefore include patients with disease not likely to trigger treatment-seeking behavior. This study based in Iquitos, Peru used the Quality of Wellbeing Scale-Self Administered, a survey that enquires about not only physical health, but also psychological health, self-care, mobility, and usual social activities, and rates HRQoL between 0 (death) and 1 (optimum function), to evaluate the impact of dengue on HRQoL. In order to enroll treatment and non treatment-seeking participants, three modalities of participant recruitment were used. In addition to clinic and community-based febrile surveillance, a contact-cluster methodology was also employed to identify infected individuals less likely to seek treatment. We measured changes in HRQoL and identified common areas of health impairment in 73 virologically confirmed dengue cases at 3 time points during the participant\u27s illness; the early-acute (days 0-6 post symptom onset), late-acute (days 7-20), and convalescent illness phases (days 21 +). Participants reported HRQoL related impairments at significantly higher frequency during the early-acute versus convalescent illness phase (Fisher\u27s exact: P\u3c0.01). There was substantial heterogeneity in scores during each illness phase with median scores in the early-acute, late-acute and convalescent phases of 0.56 (IQR: 0.41-0.64), 0.70 (IQR: 0.57-0.94), and 1 (IQR: 0.80-1.00), respectively. In all illness phases participants recruited in clinics had on average the lowest HRQoL scores where as those recruited in the contact clusters had the highest. Only 1 individual who was recruited in the contact-clusters had no reduction in HRQoL score during their illness. These data illustrate that dengue should be considered as a disease that may have significant implications for not only physical health but also psychological health and social functioning. The impact of dengue on the HRQoL of non-treatment-seeking individuals, although lower than the impact among treatment-seeking individuals, is not necessarily trivial

The impact of dengue illness on social distancing and caregiving behavior

Background Human mobility among residential locations can drive dengue virus (DENV) transmission dynamics. Recently, it was shown that individuals with symptomatic DENV infection exhibit significant changes in their mobility patterns, spending more time at home during illness. This change in mobility is predicted to increase the risk of acquiring infection for those living with or visiting the ill individual. It has yet to be considered, however, whether social contacts are also changing their mobility, either by socially distancing themselves from the infectious individual or increasing contact to help care for them. Social, or physical, distancing and caregiving could have diverse yet important impacts on DENV transmission dynamics; therefore, it is necessary to better understand the nature and frequency of these behaviors including their effect on mobility. Methodology and principal findings Through community-based febrile illness surveillance and RT-PCR infection confirmation, 67 DENV positive (DENV+) residents were identified in the city of Iquitos, Peru. Using retrospective interviews, data were collected on visitors and home-based care received during the illness. While 15% of participants lost visitors during their illness, 22% gained visitors; overall, 32% of all individuals (particularly females) received visitors while symptomatic. Caregiving was common (90%), particularly caring by housemates (91%) and caring for children (98%). Twenty-eight percent of caregivers changed their behavior enough to have their work (and, likely, mobility patterns) affected. This was significantly more likely when caring for individuals with low “health-related quality of well-being” during illness (Fisher’s Exact, p = 0.01). Conclusions/Significance Our study demonstrates that social contacts of individuals with dengue modify their patterns of visitation and caregiving. The observed mobility changes could impact a susceptible individual’s exposure to virus or a presymptomatic/clinically inapparent individual’s contribution to onward transmission. Accounting for changes in social contact mobility is imperative in order to get a more accurate understanding of DENV transmission

Planetary Nebulae as Standard Candles. XII. Connecting the Population I and Population II Distance Scales

We report the results of [OIII] lambda 5007 surveys for planetary nebulae (PNe) in NGC 2403, 3115, 3351, 3627, 4258, and 5866. Using on-band/off-band [OIII] and H-alpha images, we identify samples of PNe in these galaxies and derive distances using the planetary nebula luminosity function (PNLF). We then combine these measurements with previous data to compare the PNLF, Cepheid, and surface brightness fluctuation (SBF) distance scales. We use a sample of 13 galaxies to show that the absolute magnitude of the PNLF cutoff is fainter in small, low-metallicity systems, but the trend is well modelled theoretically. When this dependence is removed, the scatter between the Cepheid and PNLF distances becomes consistent with the internal errors of the methods and independent of any obvious galaxy parameter. We then use the data to recalibrate the zero point of the PNLF distance scale. We use a sample of 28 galaxies to show that the scatter between the PNLF and SBF distance measurements agrees with that predicted from the techniques' internal errors, and that no systematic trend exists between the distance residuals and stellar population. However, we find the PNLF and SBF methods have a significant scale offset: Cepheid-calibrated PNLF distances are, on average, ~0.3 mag smaller than Cepheid-calibrated SBF distances. We discuss the possible causes of this offset, and suggest that internal extinction in the bulges of the SBF calibration galaxies is the principle cause of the discrepancy. If this is correct, the SBF-based Hubble Constant must be increased by ~7%. We use our distance to NGC 4258 to argue that the short distance scale to the LMC is correct, and that the global Hubble Constant inferred from the HST Key Project should be increased by 8 +/- 3% to H_0 = 78 +/- 7 km/s/Mpc. (abridged)Comment: 38 pages, 9 figures included, accepted for publication in the Astrophysical Journa