Charles A. Kuehn

A reverberation-based mass for the central black hole in NGC 4151

We have undertaken a new ground-based monitoring campaign to improve the estimates of the mass of the central black hole in NGC 4151. We measure the lag time of the broad H? line response compared to the optical continuum at 5100 ? and find a lag of 6.6 days. We combine our data with the recent reanalysis of UV emission lines by Metzroth and coworkers to calculate a weighted mean of the black hole mass, MBH = (4.57) ? 107 M?. The absolute calibration of the black hole mass is based on normalization of the AGN black hole mass-stellar velocity dispersion (MBH-?*) relationship to that of quiescent galaxies by Onken and coworkers. The scatter in the MBH-?* relationship suggests that reverberation-mapping-based mass measurements are typically uncertain by a factor of 3-4.

The Mass of the Black Hole in the Seyfert 1 Galaxy NGC 4593 from Reverberation Mapping

We present new observations leading to an improved black hole mass estimate for the Seyfert 1 galaxy NGC 4593 as part of a reverberation-mapping campaign conducted at the MDM Observatory. Cross-correlation analysis of the Hβ emission-line light curve with the optical continuum light curve reveals an emission-line time delay of τcent = 3.73 ± 0.75 days. By combining this time delay with the Hβ line width, we derive a central black hole mass of MBH = (9.8 ± 2.1) × 106 M☉, an improvement in precision of a factor of several over past results.

NGC 5548 in a Low-Luminosity State: Implications for the Broad-Line Region

We describe results from a new ground-based monitoring campaign on NGC 5548, the best-studied reverberation-mapped AGN. We find that it was in the lowest luminosity state yet recorded during a monitoring program, namely L5100 = 4.7 × 1042 ergs s-1. We determine a rest-frame time lag between flux variations in the continuum and the Hβ line of 6.3 days. Combining our measurements with those of previous campaigns, we determine a weighted black hole mass of MBH = 6.54 × 107 M☉ based on all broad emission lines with suitable variability data. We confirm the previously discovered virial relationship between the time lag of emission lines relative to the continuum and the width of the emission lines in NGC 5548, which is the expected signature of a gravity-dominated broad-line region. Using this lowest luminosity state, we extend the range of the relationship between the luminosity and the time lag in NGC 5548 and measure a slope that is consistent with α = 0.5, the naive expectation for the broad-line region for an assumed form of r ∝ Lα. This value is also consistent with the slope recently determined by Bentz et al. for the population of reverberation-mapped AGNs as a whole.

Variable Stars in the Newly Discovered Milky Way Satellite in Bootes

We present V, I light curves for 12 variable stars identified in the newly discovered satellite of the Milky Way in the Bootes constellation. Our sample includes 11 RR Lyrae stars (five first overtone, five fundamental mode, and one double-mode pulsator) and one long-period variable close to the galaxy red giant branch tip. The RR Lyrae stars trace very well the average V luminosity of the galaxy horizontal branch, leading to a true distance modulus for the galaxy of μ0 = 19.11 ± 0.08 mag for an assumed metal abundance of [Fe/H] = -2.5 and for E(B - V) = 0.02 mag. Average periods are Pab = 0.69 days and Pc = 0.37 days for ab- and c-type RR Lyrae stars, respectively, making Bootes the second pure Oosterhoff type II (OoII) dSph after Ursa Minor. The location of the double-mode RR Lyrae (RRd) in the Petersen diagram is consistent with RRd stars in OoII clusters and corresponds to an intrinsic luminosity of log L/ log L☉ = 1.72 (for Z = 10-4 and M = 0.80 M☉) according to nonlinear convective pulsation models.

PROPERTIES OF OSCILLATION MODES IN SUBGIANT STARS OBSERVED BY KEPLER

Mixed modes seen in evolved stars carry information on their deeper layers that can place stringent constraints on their physics and on their global properties (mass, age, etc.). In this study, we present a method to identify and measure all oscillatory mode characteristics (frequency, height, width). Analyzing four subgiant stars, we present the first measure of the effect of the degree of mixture on the l = 1 mixed mode characteristics. We also show that some stars have measurable l = 2 mixed modes and discuss the interest of their measure to constrain the deeper layers of stars.

ON THE SIZE OF THE Fe II-EMITTING REGION IN THE AGN ARAKELIAN 120

We present a reverberation analysis of the strong, variable optical Fe II emission bands in the spectrum of Ark 120, a low-redshift AGN which is one of the best candidates for such a study. On timescales of several years the Fe II line strengths follow the variations in the continuum strength. However, we are unable to measure a clear reverberation lag time for these Fe II lines on any timescale. This is due to the very broad and flat-topped nature of the Fe II cross-correlation functions, as compared to the Hβ response, which is much more sharply localized in time. Although there is some suggestion in the light curve of a 300 day response time, our statistical analysis does not pick up such a feature. We conclude that the optical Fe II emission does not come from a photoionization-powered region similar in size to the Hβ-emitting region, but we cannot say for sure where it does come from. Our results are generally consistent either with emission from a photoionized region several times larger than the Hβ zone, or with emission from gas heated by some other means, perhaps responding only indirectly to the continuum variations.

Variable Stars in the Newly Discovered Milky Way Dwarf Spheroidal Satellite Canes Venatici I

We have identified 23 RR Lyrae stars and three possible anomalous Cepheids (ACs) among 84 candidate variables in the recently discovered Canes Venatici I (CVn I) dwarf spheroidal galaxy. The mean period of 18 RRab type stars, --> Pab = 0.60 ± 0.01 days, and the location of these stars in the period-amplitude diagram suggest that CVn I is likely an Oosterhoff-intermediate system. The average apparent magnitude of the RR Lyrae stars, --> V = 22.17 ± 0.02 mag, gives a distance of 210 -->+ 7−5 kpc, for an adopted reddening -->E(B − V) = 0.03 mag. We present a B, V color-magnitude diagram (CMD) of CVn I that reaches -->V ~ 25 mag, and shows that the galaxy has a mainly old stellar population with a metal abundance near [Fe/H] = –2.0 dex. The width of the red giant branch and the location of the candidate ACs on the CMD may indicate that the galaxy hosts a complex stellar population with stars from ~13 Gyr to as young as ~0.6 Gyr.

On the Newly Discovered Canes Venatici II dSph Galaxy

We report on the detection of variable stars in the Canes Venatici II (CVn II) dwarf spheroidal galaxy, a new satellite of the Milky Way recently discovered by the Sloan Digital Sky Survey. We also present a V, -->B − V color-magnitude diagram that reaches -->V ~ 25.5 mag, showing the galaxys main-sequence turnoff at -->V ~ 24.5 mag and revealing several candidate blue straggler stars. Two RR Lyrae stars have been identified within the half-light radius of CVn II, a fundamental-mode variable (RRab) with period -->Pab = 0.743 days, and a first-overtone (RRc) RR Lyrae star with -->Pc = 0.358 days. The rather long periods of these variables along with their position on the period-amplitude diagram support an Oosterhoff type II classification for CVn II. The average apparent magnitude of the RR Lyrae stars, --> V = 21.48 ± 0.02 mag, is used to obtain a precision distance modulus of -->μ0 = 21.02 ± 0.06 mag and a corresponding distance of -->160+ 4−5 kpc, for an adopted reddening -->E(B − V) = 0.015 mag.

An Evaluation of the Membership Probability of 212 λ Boo Stars. I. A Catalogue

The literature on the λ Boo stars has grown to become somewhat heterogenous, as different authors have applied different criteria across the UV, optical, and infrared regions to determine the membership status of λ Boo candidates. We aim to clear up the confusion by consulting the literature on 212 objects that have been considered as λ Boo candidates, and subsequently evaluating the evidence in favour of their admission to the λ Boo class. We obtained new spectra of ~ 90 of these candidates and classified them on the MK system to aid in the membership evaluations. The re-evaluation of the 212 objects resulted in 64 members and 103 non-members of the λ Boo class, with a further 45 stars for which membership status is unclear. We suggest observations for each of the stars in the latter category that will allow them to be confidently included or rejected from the class. Our reclassification facilitates homogenous analysis on group members, and represents the largest collection of confirmed λ Boo stars known.

The nature of the H2-emitting gas in the Crab nebula

Understanding how molecules and dust might have formed within a rapidly expanding young supernova remnant is important because of the obvious application to vigorous supernova activity at very high redshift. In previous papers, we found that the H2 emission is often quite strong, correlates with optical low-ionization emission lines, and has a surprisingly high excitation temperature. Here we study Knot 51, a representative, bright example, for which we have available long slit optical and NIR spectra covering emission lines from ionized, neutral, and molecular gas, as well as HST visible and SOAR Telescope NIR narrow-band images. We present a series of CLOUDY simulations to probe the excitation mechanisms, formation processes and dust content in environments that can produce the observed H2 emission. We do not try for an exact match between model and observations given Knot 51s ambiguous geometry. Rather, we aim to explain how the bright H2 emission lines can be formed from within the volume of Knot 51 that also produces the observed optical emission from ionized and neutral gas. Our models that are powered only by the Crabs synchrotron radiation are ruled out because they cannot reproduce the strong, thermal H2 emission. The simulations that come closest to fitting the observations have the core of Knot 51 almost entirely atomic with the H2 emission coming from just a trace molecular component, and in which there is extra heating. In this unusual environment, H2 forms primarily by associative detachment rather than grain catalysis. In this picture, the 55 H2-emitting cores that we have previously catalogued in the Crab have a total mass of about 0.1 M_sun, which is about 5% of the total mass of the system of filaments. We also explore the effect of varying the dust abundance. We discuss possible future observations that could further elucidate the nature of these H2 knots.