Xiao Che

The structure of the broad-line region in active galactic nuclei. I. Reconstructed velocity-delay maps

We present velocity-resolved reverberation results for five active galactic nuclei. We recovered velocity-delay maps using the maximum entropy method for four objects: Mrk 335, Mrk 1501, 3C?120, and PG?2130+099. For the fifth, Mrk 6, we were only able to measure mean time delays in different velocity bins of the H? emission line. The four velocity-delay maps show unique dynamical signatures for each object. For 3C?120, the Balmer lines show kinematic signatures consistent with both an inclined disk and infalling gas, but the He II??4686 emission line is suggestive only of inflow. The Balmer lines in Mrk 335, Mrk 1501, and PG?2130+099 show signs of infalling gas, but the He II emission in Mrk 335 is consistent with an inclined disk. We also see tentative evidence of combined virial motion and infalling gas from the velocity-binned analysis of Mrk 6. The maps for 3C?120 and Mrk 335 are two of the most clearly defined velocity-delay maps to date. These maps constitute a large increase in the number of objects for which we have resolved velocity-delay maps and provide evidence supporting the reliability of reverberation-based black hole mass measurements.

COLDER AND HOTTER: INTERFEROMETRIC IMAGING OF β CASSIOPEIAE AND α LEONIS

Near-infrared interferometers have recently imaged a number of rapidly rotating A-type stars, finding levels of gravity darkening inconsistent with theoretical expectations. Here, we present new imaging of both a cooler star ??Cas (F2IV) and a hotter one ? Leo (B7V) using the CHARA array and the MIRC instrument at the H band. Adopting a solid-body rotation model with a simple gravity darkening prescription, we modeled the stellar geometric properties and surface temperature distributions, confirming that both stars are rapidly rotating and show gravity darkening anomalies. We estimate the masses and ages of these rapid rotators on L-R pol and H-R diagrams constructed for non-rotating stars by tracking their non-rotating equivalents. The unexpected fast rotation of the evolved sub-giant ??Cas offers a unique test of the stellar core-envelope coupling, revealing quite efficient coupling over the past ~0.5?Gyr. Lastly, we summarize all our interferometric determinations of the gravity darkening coefficient for rapid rotators, finding that none match the expectations from the widely used von Zeipel gravity darkening laws. Since the conditions of the von Zeipel law are known to be violated for rapidly rotating stars, we recommend using the empirically derived ??= 0.19 for such stars with radiation-dominated envelopes. Furthermore, we note that no paradigm exists for self-consistently modeling heavily gravity-darkened stars that show hot radiative poles with cool convective equators.

Interferometric radii of bright Kepler stars with the CHARA Array : θ Cygni and 16 Cygni A and B

We present the results of long-baseline optical interferom etry observations using the Precision Astronomical Visual Observations (PAVO) beam combiner at the Center for High Angular Resolution Astronomy (CHARA) Array to measure the angular sizes of three bright Kepler stars: θ Cygni, and both components of the binary system 16 Cygni. Supporting infrared observations were made with the Michigan Infrared Combiner (MIRC) and Classic beam combiner, also at the CHARA Array. We find limb-darkened angular diameters of 0.753 ± 0.009 mas for θ Cyg, 0.539 ± 0.007 mas for 16 Cyg A and 0.490 ± 0.006 mas for 16 Cyg B. The Kepler Mission has observed these stars with outstanding photometric precision, revealing the presence of solar-like oscillations. Due to the brightness of these stars the oscillations have exceptiona l signal-to-noise, allowing for detailed study through asteroseismology, and are well constrained by other observations. We have combined our interferometric diameters with Hipparcos parallaxes, spectrophotometric bolometric fluxes and the asteroseismic large frequency sep aration to measure linear radii (θ Cyg: 1.48±0.02 R⊙, 16 Cyg A: 1.22±0.02 R⊙, 16 Cyg B: 1.12±0.02 R⊙), effective temperatures (θ Cyg: 6749±44 K, 16 Cyg A: 5839±42 K, 16 Cyg B: 5809±39 K), and masses (θ Cyg: 1.37±0.04 M⊙, 16 Cyg A: 1.07±0.05 M⊙, 16 Cyg B: 1.05±0.04 M⊙) for each star with very little model dependence. The measurements presented here will provide strong constraints for future stellar modelling efforts.

A near-infrared interferometric survey of debris disc stars : III. First statistics based on 42 stars observed with CHARA/FLUOR

Context. Dust is expected to be ubiquitous in extrasolar planetary systems owing to the dynamical activity of minor bodies. Inner dust populations are, however, still poorly known because of the high contrast and small angular separation with respect to their host star, and yet, a proper characterisation of exozodiacal dust is mandatory for the design of future Earth-like planet imaging missions. Aims. We aim to determine the level of near-infrared exozodiacal dust emission around a sample of 42 nearby main sequence stars with spectral types ranging from A to K and to investigate its correlation with various stellar parameters and with the presence of cold dust belts. Methods. We use high-precision K-band visibilities obtained with the FLUOR interferometer on the shortest baseline of the CHARA array. The calibrated visibilities are compared with the expected visibility of the stellar photosphere to assess whether there is an additional, fully resolved circumstellar emission source. Results. Near-infrared circumstellar emission amounting to about 1% of the stellar flux is detected around 13 of our 42 target stars. Follow-up observations showed that one of them (eps Cep) is associated with a stellar companion, while another one was detected around what turned out to be a giant star (kap CrB). The remaining 11 excesses found around single main sequence stars are most probably associated with hot circumstellar dust, yielding an overall occurrence rate of 28 +8 % for our (biased) sample. We show that the occurrence rate of bright exozodiacal discs correlates with spectral type, K-band excesses being more frequent around A-type stars. It also correlates with the presence of detectable far-infrared excess emission in the case of solar-type stars. Conclusions. This study provides new insight into the phenomenon of bright exozodiacal discs, showing that hot dust populations are probably linked to outer dust reservoirs in the case of solar-type stars. For A-type stars, no clear conclusion can be made regarding the origin of the detected near-infrared excesses.

Resolving Vega and the inclination controversy with CHARA/MIRC

Optical and infrared interferometers definitively established that the photometric standard Vega (={alpha} Lyrae) is a rapidly rotating star viewed nearly pole-on. Recent independent spectroscopic analyses could not reconcile the inferred inclination angle with the observed line profiles, preferring a larger inclination. In order to resolve this controversy, we observed Vega using the six-beam Michigan Infrared Combiner on the Center for High Angular Resolution Astronomy Array. With our greater angular resolution and dense (u, v)-coverage, we find that Vega is rotating less rapidly and with a smaller gravity darkening coefficient than previous interferometric results. Our models are compatible with low photospheric macroturbulence and are also consistent with the possible rotational period of {approx}0.71 days recently reported based on magnetic field observations. Our updated evolutionary analysis explicitly incorporates rapid rotation, finding Vega to have a mass of 2.15{sup +0.10}{sub -0.15} M{sub Sun} and an age 700{sup -75}{sub +150} Myr, substantially older than previous estimates with errors dominated by lingering metallicity uncertainties (Z = 0.006{sup +0.003}{sub -0.002}).

In the Shadow of the Transiting Disk: Imaging epsilon Aurigae in Eclipse

Epsilon Aurigae (ε Aur) is a visually bright, eclipsing binary star system with a period of 27.1 years. The cause of each 18-month-long eclipse has been a subject of controversy for nearly 190 years because the companion has hitherto been undetectable. The orbital elements imply that the opaque object has roughly the same mass as the visible component, which for much of the last century was thought to be an F-type supergiant star with a mass of ∼15M⊙ (M⊙, mass of the Sun). The high mass-to-luminosity ratio of the hidden object was originally explained by supposing it to be a hyperextended infrared star or, later, a black hole with an accretion disk, although the preferred interpretation was as a disk of opaque material at a temperature of ∼500 K, tilted to the line of sight and with a central opening. Recent work implies that the system consists of a low-mass (2.2M⊙–3.3M⊙) visible F-type star, with a disk at 550 K that enshrouds a single B5V-type star. Here we report interferometric images that show the eclipsing body moving in front of the F star. The body is an opaque disk and appears tilted as predicted. Adopting a mass of 5.9M⊙ for the B star, we derive a mass of ∼(3.6 ± 0.7)M⊙ for the F star. The disk mass is dynamically negligible; we estimate it to contain ∼0.07M⊕ (M⊕, mass of the Earth) if it consists purely of dust.directly detected the eclipsing body, allowing us to measure theproperties of the companion. We used the MIRC four-telescopebeam combiner at the CHARA Array to obtain images of epsilonAurigae during ingress into eclipse during autumn 2009. Theseimages show the intrusion of a dark, elongated structure that re-sembles the large disk as rst discussed by Ludendor

A REVERBERATION LAG FOR THE HIGH-IONIZATION COMPONENT OF THE BROAD-LINE REGION IN THE NARROW-LINE SEYFERT 1 Mrk 335

We present the first results from a detailed analysis of photometric and spectrophotometric data on the narrow-line Seyfert 1 (NLS1) galaxy Mrk 335, collected over a 120 day span in the fall of 2010. From these data we measure the lag in the He II {lambda}4686 broad emission line relative to the optical continuum to be 2.7 {+-} 0.6 days and the lag in the H{beta}{lambda}4861 broad emission line to be 13.9 {+-} 0.9 days. Combined with the line width, the He II lag yields a black hole mass M{sub BH} = (2.6 {+-} 0.8) Multiplication-Sign 10{sup 7} M{sub Sun }. This measurement is consistent with measurements made using the H{beta}{lambda}4861 line, suggesting that the He II emission originates in the same structure as H{beta}, but at a much smaller radius. This constitutes the first robust lag measurement for a high-ionization line in an NLS1 galaxy and supports a scenario in which the He II emission originates from gas in virial motion rather than outflow.

Gas Distribution, Kinematics, and Excitation Structure in the Disks around the Classical Be Stars β Canis Minoris and ζ Tauri

Using CHARA and VLTI near-infrared spectro-interferometry with hectometric baseline lengths (up to 330 m) and with high spectral resolution (up to λ/Δλ = 12, 000), we studied the gas distribution and kinematics around two classical Be stars. The combination of high spatial and spectral resolution achieved allows us to constrain the gas velocity field on scales of a few stellar radii and to obtain, for the first time in optical interferometry, a dynamical mass estimate using the position-velocity analysis technique known from radio astronomy. For our first target star, β Canis Minoris, we model the H+K-band continuum and Brγ-line geometry with a near-critical rotating stellar photosphere and a geometrically thin equatorial disk. Testing different disk rotation laws, we find that the disk is in Keplerian rotation (v(r)∝r –0.5 ± 0.1) and derive the disk position angle (140° ± 17), inclination (385 ± 1°), and the mass of the central star (3.5 ± 0.2 M ☉). As a second target star, we observed the prototypical Be star ζ Tauri and spatially resolved the Brγ emission as well as nine transitions from the hydrogen Pfund series (Pf 14-22). Comparing the spatial origin of the different line transitions, we find that the Brackett (Brγ), Pfund (Pf 14-17), and Balmer (Hα) lines originate from different stellocentric radii (R cont < R Pf < R Brγ ~ R Hα), which we can reproduce with an LTE line radiative transfer computation. Discussing different disk-formation scenarios, we conclude that our constraints are inconsistent with wind compression models predicting a strong outflowing velocity component, but support viscous decretion disk models, where the Keplerian-rotating disk is replenished with material from the near-critical rotating star.

Imaging the Algol Triple System in the H Band with the CHARA Interferometer

Algol (β Per) is an extensively studied hierarchical triple system whose inner pair is a prototype semi-detached binary with mass transfer occurring from the sub-giant secondary to the main-sequence primary. We present here the results of our Algol observations made between 2006 and 2010 at the CHARA interferometer with the Michigan Infrared Combiner in the H-band. The use of four telescopes with long baselines allows us to achieve better than 0.5 mas resolution and to unambiguously resolve the three stars. The inner and outer orbital elements, as well as the angular sizes and mass ratios for the three components, are determined independently from previous studies. We report a significantly improved orbit for the inner stellar pair with the consequence of a 15% change in the primary mass compared with previous studies. We also determine the mutual inclination of the orbits to be much closer to perpendicularity than previously established. State-of-the-art image reconstruction algorithms are used to image the full triple system. In particular an image sequence of 55 distinct phases of the inner pair orbit is reconstructed, clearly showing the Roche-lobe-filling secondary revolving around the primary, with several epochs corresponding to the primary and secondary eclipses.

Toward Direct Detection of Hot Jupiters with Precision Closure Phase: Calibration Studies and First Results from the CHARA Array

Direct detection of thermal emission from nearby hot Jupiters has greatly advanced our knowledge of extrasolar planets in recent years. Since hot Jupiter systems can be regarded as analogs of high-contrast binaries, ground-based infrared long-baseline interferometers have the potential to resolve them and detect their thermal emission with precision closure phase—a method that is immune to the systematic errors induced by the Earths atmosphere. In this work, we present closure-phase studies toward direct detection of nearby hot Jupiters using the CHARA interferometer array outfitted with the MIRC instrument. We carry out closure-phase simulations and conduct a large number of observations for the best candidate υ And. Our experiments suggest that the method is feasible with highly stable and precise closure phases. However, we also find much larger systematic errors than expected in the observations, most likely caused by dispersion across different wavelengths. We find that using higher spectral resolution modes (e.g., R ¼ 150) can significantly reduce the systematics. By combining all cali- brators in an observing run together, we are able to roughly recalibrate the lower spectral resolution data, allowing us to obtain upper limits of the star-planet contrast ratios of υ And b across the H band. The data also allow us to get a refined stellar radius of 1:625 � 0:011 R⊙. Our best upper limit corresponds to a contrast ratio of 2:1 × 10 3 :1 with 90% confidence level at 1.52 μm, suggesting that we are starting to have the capability of constraining atmospheric models of hot Jupiters with interferometry. With recent and upcoming improvements of CHARA/MIRC, the pros- pect of detecting emission from hot Jupiters with closure phases is promising.