Photometric Variability Among the Brightest Asymptotic Giant Branch Stars Near the Center of M32

Deep K' images with 0.1 arcsec angular resolution, obtained with ALTAIR+NIRI on Gemini North, are used to investigate photometric variablity among the brightest asymptotic giant branch (AGB) stars in the central regions of M32. Based on a comparison with brightnesses obtained from the K-band data discussed by Davidge et al. (2000, ApJ, 545, L89), it is concluded that (1) at least 60% of bright AGB stars near the center of M32 are photometrically variable, and (2) the amplitudes of the light variations are similar to those of long period variables in the Galactic bulge. We do not find evidence for a population of large amplitude variables, like those detected by IRAS in the Galactic bulge. The technique discussed here may prove useful for conducting an initial reconnaisance of photometric variability among AGB stars in spheroids in the Virgo cluster and beyond, where the required long exposure times may restrict observations to only a few epochs.Comment: 8 pages of text, 3 postscript figures. ApJ (letters) in pres

Astrometric performance of the Gemini multi-conjugate adaptive optics system in crowded fields

The Gemini Multi-conjugate adaptive optics System (GeMS) is a facility instrument for the Gemini-South telescope. It delivers uniform, near-diffraction-limited image quality at near-infrared wavelengths over a 2 arcminute field of view. Together with the Gemini South Adaptive Optics Imager (GSAOI), a near-infrared wide field camera, GeMS/GSAOI's combination of high spatial resolution and a large field of view will make it a premier facility for precision astrometry. Potential astrometric science cases cover a broad range of topics including exo-planets, star formation, stellar evolution, star clusters, nearby galaxies, black holes and neutron stars, and the Galactic center. In this paper, we assess the astrometric performance and limitations of GeMS/GSAOI. In particular, we analyze deep, mono-epoch images, multi-epoch data and distortion calibration. We find that for single-epoch, un-dithered data, an astrometric error below 0.2 mas can be achieved for exposure times exceeding one minute, provided enough stars are available to remove high-order distortions. We show however that such performance is not reproducible for multi-epoch observations, and an additional systematic error of ~0.4 mas is evidenced. This systematic multi-epoch error is the dominant error term in the GeMS/GSAOI astrometric error budget, and it is thought to be due to time-variable distortion induced by gravity flexure.Comment: 16 pages, 22 figures, accepted for publication in MNRA

First test of a high voltage feedthrough for liquid Argon TPCs connected to a 300 kV power supply

Voltages above a hundred kilo-volt will be required to generate the drift field of future very large liquid Argon Time Projection Chambers. The most delicate component is the feedthrough whose role is to safely deliver the very high voltage to the cathode through the thick insulating walls of the cryostat without compromising the purity of the argon inside. This requires a feedthrough that is typically meters long and carefully designed to be vacuum tight and have small heat input. Furthermore, all materials should be carefully chosen to allow operation in cryogenic conditions. In addition, electric fields in liquid argon should be kept below a threshold to reduce risks of discharges. The combination of all above requirements represents significant challenges from the design and manufacturing perspective. In this paper, we report on the successful operation of a feedthrough satisfying all the above requirements. The details of the feedthrough design and its manufacturing steps are provided. Very high voltages up to unprecedented voltages of -300 kV could be applied during long periods repeatedly. A source of instability was observed, which was specific to the setup configuration which was used for the test and not due to the feedthrough itself.Comment: 13 pages, 9 figure

The Peak Brightness and Spatial Distribution of AGB Stars Near the Nucleus of M32

The bright stellar content near the center of the Local Group elliptical galaxy M32 is investigated with 0.12 arcsec FWHM H and K images obtained with the Gemini Mauna Kea telescope. Stars with K = 15.5, which are likely evolving near the tip of the asymptotic giant branch (AGB), are resolved to within 2 arcsec of the nucleus, and it is concluded that the peak stellar brightness near the center of M32 is similar to that in the outer regions of the galaxy. Moreover, the projected density of bright AGB stars follows the visible light profile to within 2 arcsec of the nucleus, indicating that the brightest stars are well mixed throughout the galaxy. Thus, there is no evidence for an age gradient, and the radial variations in spectroscopic indices and ultraviolet colors that have been detected previously must be due to metallicity and/or some other parameter. We suggest that either the bright AGB stars formed as part of a highly uniform and coherent galaxy-wide episode of star formation, or they originated in a separate system that merged with M32.Comment: 9 pages of text, 3 figures. ApJ (Letters) in pres

Deep ALTAIR + NIRI Imaging of the Disk and Bulge of M31

Deep J, H, and K' images, recorded with the ALTAIR adaptive optics system and NIRI imager on Gemini North, are used to probe the stellar content of the disk and bulge of the Local Group galaxy M31. With FWHM near 0.08 arcsec in K, these are the highest angular resolution near-infrared images yet obtained of this galaxy. Four fields that sample M31 at galactocentric radii of 62, 9, 4, and 2 arcmin were observed. The RGB-tip occurs between K = 17.0 and 17.2, and the color of the RGB in the field closest to the center of M31 is consistent with that of NGC 6528. After accounting for random photometric errors, the upper RGB in each field has a width on the (K, J-K) CMD that is consistent with a +/- 0.5 dex dispersion in [Fe/H], in rough agreement with what is seen in other disk and spheroid fields in M31. A population of very bright red stars, which we identify as C stars, are seen in the three fields that are closest to the center of M31. The spatial distribution of these objects suggests that they are well mixed throughout this part of M31, and so likely did not form in a compact region near the galactic nucleus, but more probably formed in the inner disk. We speculate that these C stars may be the most luminous members of the intermediate age population that has been detected previously in studies of the integrated spectrum of the central regions of M31.Comment: 36 pages of text + 16 eps figures; Astronomical Journal in pres

Near-Infrared, Kilosecond Variability of the Wisps and Jet in the Crab Pulsar Wind Nebula

We present a time-lapse sequence of 20 near-infrared (J- and K'-band) snapshots of the central 20x20 arcsec of the Crab pulsar wind nebula, taken at subarcsecond resolution with the Hokupa'a/QUIRC adaptive optics camera on the Gemini North Telescope, and sampled at intervals of 10 min and 24 hr. It is observed that the equatorial wisps and polar knots in the termination shock of the pulsar wind appear to fluctuate in brightness on kilosecond time-scales. Maximum flux variations of +/-24 +/-4 and +/-14 +/-4 per cent relative to the mean (in 1.2 ks) are measured for the wisps and knots respectively, with greatest statistical significance in J band where the nebula background is less prominent. The J and K' flux densities imply different near-infrared spectra for the nonthermal continuum emission from the wisps and outermost polar knot (sprite), giving F_nu ~ nu^{-0.56+/-0.12} and F_nu ~ nu^{-0.21+/-0.13) respectively. The data are compared with existing optical and UV photometry and applied to constrain theories of the variability of the wisps (relativistic ion-cyclotron instability) and knots (relativistic fire hose instability).Comment: 10 pages, 9 figures, to be published in The Astrophysical Journa

Characterizing the Adaptive Optics Off-Axis Point-Spread Function - I: A Semi-Empirical Method for Use in Natural-Guide-Star Observations

Even though the technology of adaptive optics (AO) is rapidly maturing, calibration of the resulting images remains a major challenge. The AO point-spread function (PSF) changes quickly both in time and position on the sky. In a typical observation the star used for guiding will be separated from the scientific target by 10" to 30". This is sufficient separation to render images of the guide star by themselves nearly useless in characterizing the PSF at the off-axis target position. A semi-empirical technique is described that improves the determination of the AO off-axis PSF. The method uses calibration images of dense star fields to determine the change in PSF with field position. It then uses this information to correct contemporaneous images of the guide star to produce a PSF that is more accurate for both the target position and the time of a scientific observation. We report on tests of the method using natural-guide-star AO systems on the Canada-France-Hawaii Telescope and Lick Observatory Shane Telescope, augmented by simple atmospheric computer simulations. At 25" off-axis, predicting the PSF full width at half maximum using only information about the guide star results in an error of 60%. Using an image of a dense star field lowers this error to 33%, and our method, which also folds in information about the on-axis PSF, further decreases the error to 19%.Comment: 29 pages, 9 figures, accepted for publication in the PAS

Holographic Imaging of Crowded Fields: High Angular Resolution Imaging with Excellent Quality at Very Low Cost

We present a method for speckle holography that is optimised for crowded fields. Its two key features are an iterativ improvement of the instantaneous Point Spread Functions (PSFs) extracted from each speckle frame and the (optional) simultaneous use of multiple reference stars. In this way, high signal-to-noise and accuracy can be achieved on the PSF for each short exposure, which results in sensitive, high-Strehl re- constructed images. We have tested our method with different instruments, on a range of targets, and from the N- to the I-band. In terms of PSF cosmetics, stability and Strehl ratio, holographic imaging can be equal, and even superior, to the capabilities of currently available Adaptive Optics (AO) systems, particularly at short near-infrared to optical wavelengths. It outperforms lucky imaging because it makes use of the entire PSF and reduces the need for frame selection, thus leading to higher Strehl and improved sensitivity. Image reconstruction a posteriori, the possibility to use multiple reference stars and the fact that these reference stars can be rather faint means that holographic imaging offers a simple way to image large, dense stellar fields near the diffraction limit of large telescopes, similar to, but much less technologically demanding than, the capabilities of a multi-conjugate adaptive optics system. The method can be used with a large range of already existing imaging instruments and can also be combined with AO imaging when the corrected PSF is unstable.Comment: Accepted for publication in MNRAS on 15 Nov 201

Excited state absorption : a key phenomenon for the improvement of biphotonic based optical limiting at telecommunication wavelengths

International audienceSpectroscopic properties, two-photon absorption (TPA) and excited state absorption (ESA), of two organic cyanine dyes and of a ruthenium based organometallic cyanine are compared in order to rationalize their similar ns-optical power limiting (OPL) efficiency in the telecommunication wavelength range. The TPA contribution to the ns-OPL behavior is higher for both organic cyanines, while the main process is a TPA-induced ESA in the case of the organometallic system, in which the ruthenium induces a broadening of the NIR-ESA band and resulting in a strong spectral overlap between TPA and ESA spectra

The nature of the Galactic Center source IRS 13 revealed by high spatial resolution in the infrared

High spatial resolution observations in the 1 to 3.5 micron region of the Galactic Center source known historically as IRS 13 are presented. They include ground-based adaptive optics images in the H, Kp (2.12/0.4 micron) and L bands, NICMOS data in filters between 1.1 and 2.2 micron, and integral field spectroscopic data from BEAR, an Imaging FTS, in the HeI 2.06 micron and the Br$\gamma$ line regions. Analysis of all these data provides a completely new picture of the main component, IRS 13E, which appears as a cluster of seven individual stars within a projected diameter of ~0.5'' (0.02 pc). The brightest sources, 13E1, 13E2, 13E3 (a binary), and 13E4, are all massive stars, 13E1 a blue object, with no detected emission line while 13E2 and 13E4 are high-mass emission line stars. 13E2 is at the WR stage and 13E4 a massive O-type star. 13E3A and B are extremely red objects, proposed as other examples of dusty WR stars. All these sources have a common westward proper motion. 13E5, is a red source similar to 13E3A/B. This concentration of comoving massive hot stars, IRS 13E, is proposed as the remaining core of a massive star cluster, which could harbor an intermediate-mass black hole (IMBH) of ~1300 M_sol. This detection plays in favor of a scenario in which the helium stars and the other hot stars in the central pc originate from the stripping of a massive cluster formed several tens of pc from the center. The detection of a discrete X-ray emission (Baganoff et al. 2003) at the IRS~13 position is examined in this context.Comment: 14 pages, 6 figures (3 in color), LaTeX2e, accepted in A&