J. R. Gerke

A STUDY OF CEPHEIDS IN M81 WITH THE LARGE BINOCULAR TELESCOPE (EFFICIENTLY CALIBRATED WITH HUBBLE SPACE TELESCOPE)

We identify and phase a sample of 107 Cepheids with 10 days < P < 100 days in M81 using the Large Binocular Telescope and calibrate their B, V, and I mean magnitudes with archival Hubble Space Telescope (HST) data. The use of a ground-based telescope to identify and phase the Cepheids and HST only for the final calibration reduces the demand on this highly oversubscribed spacecraft by nearly an order of magnitude and yields period-luminosity (PL) relations with dispersions comparable to the best LMC samples. We fit the sample using the OGLE-II LMC PL relations and are unable to find a self-consistent distance for different band combinations or radial locations within M81. We can do so after adding a radial dependence to the PL zero point that corresponds to a luminosity dependence on metallicity of γμ = –0.56 ± 0.36 mag dex–1. We find marginal evidence for a shift in color as a function of metallicity, distinguishable from the effects of extinction, of γ2 = +0.07 ± 0.03 mag dex–1. We find a distance modulus for M81, relative to the LMC, of μM81 – μLMC = 9.39 ± 0.14 mag, including uncertainties due to the metallicity corrections. This corresponds to a distance to M81 of 3.6 ± 0.2 Mpc, assuming an LMC distance modulus of 18.41 mag. We carry out a joint analysis of M81 and NGC 4258 Cepheids and simultaneously solve for the distance of M81 relative to NGC 4258 and the metallicity corrections. Given the current data, the uncertainties of such joint fits are dominated by the relative metallicities and the abundance gradients rather than by measurement errors of the Cepheid magnitudes or colors. We find μM81 – μLMC = 9.40+0.15 –0.11 mag, μN4258 – μLMC = 11.08+0.21 –0.17 mag, and μN4258 – μM81 = 1.68 ± 0.08 mag and metallicity effects on luminosity and color of γμ = –0.62+0.31 –0.35 mag dex–1 and γ2 = 0.01 ± 0.01 mag dex–1. Quantitative analyses of Cepheid distances must take into account both the metallicity dependencies of the Cepheids and the uncertainties in the abundance estimates.

The search for failed supernovae with the Large Binocular Telescope: confirmation of a disappearing star

We present Hubble Space Telescope imaging confirming the optical disappearance of the failed supernova (SN) candidate identified by Gerke, Kochanek & Stanek. This ∼25 M⊙ red supergiant experienced a weak ∼10^6 L⊙ optical outburst in 2009 and is now at least 5 mag fainter than the progenitor in the optical. The mid-IR flux has slowly decreased to the lowest levels since the first measurements in 2004. There is faint (2000–3000 L⊙) near-IR emission likely associated with the source. We find the late-time evolution of the source to be inconsistent with obscuration from an ejected, dusty shell. Models of the spectral energy distribution indicate that the remaining bolometric luminosity is >6 times fainter than that of the progenitor and is decreasing as ∼t^(−4/3). We conclude that the transient is unlikely to be an SN impostor or stellar merger. The event is consistent with the ejection of the envelope of a red supergiant in a failed SN and the late-time emission could be powered by fallback accretion on to a newly formed black hole. Future IR and X-ray observations are needed to confirm this interpretation of the fate for the star.

A NEAR-INFRARED SURVEY OF THE INNER GALACTIC PLANE FOR WOLF-RAYET STARS. II. GOING FAINTER: 71 MORE NEW W-R STARS

We are continuing a J, K and narrowband imaging survey of 300 deg2 of the plane of the Galaxy, searching for new Wolf-Rayet (W-R) stars. Our survey spans 150° in Galactic longitude and reaches 1° above and below the Galactic plane. The survey has a useful limiting magnitude of K = 15 over most of the observed Galactic plane, and K = 14 (due to severe crowding) within a few degrees of the Galactic center. Thousands of emission-line candidates have been detected. In spectrographic follow-ups of 146 relatively bright W-R star candidates, we have re-examined 11 previously known WC and WN stars and discovered 71 new W-R stars, 17 of type WN and 54 of type WC. Our latest image analysis pipeline now picks out W-R stars with a 57% success rate. Star subtype assignments have been confirmed with the K-band spectra and distances approximated using the method of spectroscopic parallax. Some of the new W-R stars are among the most distant known in our Galaxy. The distribution of these new W-R stars is beginning to trace the locations of massive stars along the distant spiral arms of the Milky Way.

The search for failed supernovae with the Large Binocular Telescope: constraints from 7 yr of data

We report updated results for the first 7 yr of our programme to monitor 27 galaxies within 10 Mpc using the Large Binocular Telescope to search for failed supernovae (SNe) – core collapses of massive stars that form black holes without luminous SNe. In the new data, we identify no new compelling candidates and confirm the existing candidate. Given the six successful core-collapse SNe in the sample and one likely failed SN, the implied fraction of core collapses that result in failed SNe is f=0.14^(+0.33)_(−0.10) at 90 per cent confidence. If the current candidate is a failed SN, the fraction of failed SN naturally explains the missing high-mass red supergiants SN progenitors and the black hole mass function. If the current candidate is ultimately rejected, the data imply a 90 per cent confidence upper limit on the failed SN fraction of f < 0.35.

Discovery of Variability of the Progenitor of SN 2011dh in M 51 Using the Large Binocular Telescope

We show that the candidate progenitor of the core-collapse SN?2011dh in M?51 (8?Mpc away) was fading by 0.039 ? 0.006?mag?yr?1 during the 3?years prior to the supernova, and that this level of variability is moderately unusual for other similar stars in M?51. While there are uncertainties about whether the true progenitor was a blue companion to this candidate, the result illustrates that there are no technical challenges to obtaining fairly high precision light curves of supernova-progenitor systems using ground-based observations of nearby (<10?Mpc)?galaxies with wide-field cameras on 8?m class telescopes. While other sources of variability may dominate, it is even possible to reach into the range of evolution rates required by the quasi-static evolution of the stellar envelope. For M?81, where we have many more epochs and a slightly longer time baseline, our formal 3? sensitivity to slow changes is presently 3?mmag?yr?1 for an MV ?8?mag star. In short, there is no observational barrier to determining whether the variability properties of stars in their last phases of evolution (post-carbon ignition) are different from earlier phases.

FINDING η CAR ANALOGS IN NEARBY GALAXIES USING Spitzer. II. IDENTIFICATION OF AN EMERGING CLASS OF EXTRAGALACTIC SELF-OBSCURED STARS

Understanding the late-stage evolution of the most massive stars such as ??Carinae is challenging because no true analogs of ??Car have been clearly identified in the Milky Way or other galaxies. In Khan et al., we utilized Spitzer IRAC images of 7 nearby ( 4?Mpc) galaxies to search for such analogs, and found 34 candidates with flat or red mid-IR spectral energy distributions. Here, in Paper?II, we present our characterization of these candidates using multi-wavelength data from the optical through the far-IR. Our search detected no true analogs of ??Car, which implies an eruption rate that is a fraction 0.01 F 0.19 of the core-collapse supernova (ccSN) rate. This is roughly consistent with each M ZAMS 70 M ? star undergoing one or two outbursts in its lifetime. However, we do identify a significant population of 18 lower luminosity (log (L/L ?) 5.5-6.0) dusty stars. Stars enter this phase at a rate that is a fraction 0.09 F 0.55 of the ccSN rate, and this is consistent with all 25 < M ZAMS < 60 M ? stars undergoing an obscured phase at most lasting a few thousand years once or twice. These phases constitute a negligible fraction of post-main-sequence lifetimes of massive stars, which implies that these events are likely to be associated with special periods in the evolution of the stars. The mass of the obscuring material is of order ~M ?, and we simply do not find enough heavily obscured stars for theses phases to represent more than a modest fraction (~10% not ~50%) of the total mass lost by these stars. In the long term, the sources that we identified will be prime candidates for detailed physical analysis with the James Webb Space Telescope.

The Cepheid distance to the maser-host galaxy NGC 4258: studying systematics with the Large Binocular Telescope

We identify and phase a sample of 81 Cepheids in the maser-host galaxy NGC 4258 using the Large Binocular Telescope (LBT), and obtain calibrated mean magnitudes in up to 4 filters for a subset of 43 Cepheids using archival HST data. We employ 3 models to study the systematic effects of extinction, the assumed extinction law, and metallicity on the Cepheid distance to NGC 4258. We find a correction to the Cepheid colors consistent with a grayer extinction law in NGC 4258 compared to the Milky Way (

THE MID-IR CONTRIBUTION OF DUST-ENSHROUDED STARS IN SIX NEARBY GALAXIES

R_V =4.9

The search for failed supernovae with the Large Binocular Telescope: first candidates

), although we believe this is indicative of other systematic effects. If we combine our Cepheid sample with previously known Cepheids, we find a significant metallicity adjustment to the distance modulus of

THE MASSIVE STAR POPULATION IN M101. III. SPECTRA AND PHOTOMETRY OF THE LUMINOUS AND VARIABLE STARS

\gamma_1 = -0.61 \pm 0.21