Two New Tests of the Metallicity Sensitivity of the Cepheid Period-Luminosity Relation (The Leavitt Law)

We undertake a new test of the metallicity sensitivity of the Leavitt Law for Classical Cepheids. We derive an empirical calibration of the apparent luminosities of Cepheids as measured from the optical through the mid-infrared (0.45-8.0um) as a function of spectroscopic [Fe/H] abundances of individual Cepheids in the Large Magellanic Cloud from Romaniello et al. (2008). The cumulative trend over the entire wavelength range shows a nearly monotonic behavior. The sense of the trend is consistent with differential line-blanketing in the optical, leading to stars of high metallicity being fainter in the optical. This is followed by a reversal in the trend at longer wavelengths, with the cross-over occurring near the K band at about 2.2um, consistent with a subsequent redistribution of energy resulting in a mild brightening of Cepheids (with increased metallicity) at mid-infrared wavelengths. This conclusion agrees with that of Romaniello et al. based on a differential comparison of the mean V- and K-band Leavitt Laws for the Galaxy, SMC and LMC, but is opposite in sign to most other empirical tests of the sensitivity of Cepheid distances to mean [O/H] HII region abundances. We also search for a correlation of Cepheid host-galaxy metallicity with deviations of the galaxy's Cepheid distance from that predicted from a pure Hubble flow. Based on Cepheid distances to 26 nearby galaxies in the local flow, only a very weak signal is detected giving Dmu_o = -0.17 (+/- 0.31) ([O/H] - 8.80) - 0.21 (+/-0.10). This is in agreement with previous determinations, but statistically inconclusive.Comment: Accepted to the Astrophysical Journal. 8 pages, 5 figures, 1 tabl

Competency-based assessment for the training of PhD students and early-career scientists.

The training of PhD students and early-career scientists is largely an apprenticeship in which the trainee associates with an expert to become an independent scientist. But when is a PhD student ready to graduate, a postdoctoral scholar ready for an independent position, or an early-career scientist ready for advanced responsibilities? Research training by apprenticeship does not uniformly include a framework to assess if the trainee is equipped with the complex knowledge, skills and attitudes required to be a successful scientist in the 21st century. To address this problem, we propose competency-based assessment throughout the continuum of training to evaluate more objectively the development of PhD students and early-career scientists. © 2018, Verderame et al

Cepheid and Tip of the Red Giant Branch Distances To the Dwarf Irregular Galaxy IC10

We present color-magnitude diagrams and luminosity functions of stars in the nearby galaxy IC 10, based on VI CCD photometry acquired with the COSMIC prime-focus camera on the Palomar 5m telescope. The apparent I-band luminosity function of stars in the halo of IC 10 shows an identifiable rise at I~21.7 mag. This is interpreted as being the tip of the red giant branch (TRGB) at M_V~-4 mag. Since IC 10 is at a very low Galactic latitude, its foreground extinction is expected to be high and the uncertainty associated with that correction is the largest contributor to the error associated with its distance determination. Multi-wavelength observations of Cepheid variable stars in IC 10 give a Population I distance modulus of 24.1 +- 0.2 mag, which corresponds to a linear distance of 660 +- 66 kpc for a total line-of-sight reddening of E(B-V) = 1.16 +- 0.08 mag, derived self-consistently from the Cepheid data alone. Applying this Population I reddening to the Population II halo stars gives a TRGB distance modulus of 23.5 +- 0.2 mag, corresponding to 500 +- 50 kpc. We consider this to be a lower limit on the TRGB distance. Reconciling the Cepheid and TRGB distances would require that the reddening to the halo is $\Delta$E(B-V) = 0.31 mag lower than that into the main body of the galaxy. This then suggests that the Galactic extinction in the direction of IC10 is (B-V) ~ 0.85