Markus E. Kasper

The Very Red Afterglow of GRB 000418: Further Evidence for Dust Extinction in a Gamma-Ray Burst Host Galaxy*

We report near-infrared and optical follow-up observations of the afterglow of the GRB 000418 start- ing 2.5 days after the occurrence of the burst and extending over nearly 7 weeks. GRB 000418 represents the second case for which the afterglow was initially identi—ed by observations in the near-infrared. During the —rst 10 days its R-band afterglow was well characterized by a single power-law decay with a slope of 0.86. However, at later times the temporal evolution of the afterglow —attens with respect to a simple power-law decay. Attributing this to an underlying host galaxy, we —nd its magnitude to be R \ 23.9 and an intrinsic afterglow decay slope of 1.22. The afterglow was very red with R(K B 4 mag. The observations can be explained by an adiabatic, spherical —reball solution and a heavy reddening due to dust extinction in the host galaxy. This supports the picture that (long) bursts are associated with events in star-forming regions. Subject heading: gamma rays: bursts

ALFA: Adaptive Optics for the Calar Alto Observatory Optics, Control Systems, and Performance

The adaptive optics system ALFA differs in some aspects from systems like ADONIS and PUEO which have delivered scientific results since years. Interchangeable lenslet arrays with different numbers of subapertures and a deformable mirror with many more actuators than the number of corrected modesresult in some peculiarities in the calibration of the system and the reconstruction of incident wavefronts.We describe the design of ALFAs optics and its modal control architecture with a focus on a comparative study of the performance of different mode sets used to correct the wavefront aberrations. An outlook on our plans to improve and simplify the use of ALFA is given.The last section is dedicated to issues related to observing with ALFA in its present state. Expected Strehl ratios for different seeing conditions and guide star magnitudes are summarized in a table. AO observations in general, direct imaging and doing spectroscopywith ALFA in particular are discussed.

Spatially Resolved Imaging Spectroscopy of T Tauri

The small separation of the individual stellar components in the T Tau system has prevented spectroscopy of the individual components in the near-infrared (NIR) in the past. We present NIR data (H and K band) on this object taken using the Adaptive Optics with a Laser for Astronomy (ALFA) system in combination with the 3D integral field spectrograph. Except for the Brackett series seen in emission, the NIR spectra of T Tau S appear featureless, suggesting that warm dust dominates the radiation. No rovibrational lines of molecular hydrogen were found in close vicinity to the stars. The Brackett line emission from T Tau N and S could not be resolved with the 3.5 m telescope, implying that it arises within 6 AU from the stars. The line ratios between Brγ (2.166 μm) and Br10 (1.736 μm) toward T Tau N and T Tau S are similar, suggesting similar selective extinctions toward the respective emitting regions. Our results are consistent with a model that describes T Tau S as a pre-main-sequence star surrounded by a small edge-on disk, leaving the polar regions relatively unobscured. We present numerical simulations which support this model.

ALFA: the MPIA/MPE adaptive optics with a laser for astronomy project

The Max-Planck-Institutes for Astronomy and for Extraterrestrial Physics (MPE) have recently installed a laser guide star (LGS) adaptive optics (AO) system at the 3.5m telescope on Calar Alto in Spain. The AO system consists of a Shack-Hartmann sensor, a deformable mirror with 97 actuators, and a wave-front processor that allows closed loop operations of up to 1200 Hz. As a first step we closed the high order AO loop on bright natural guide stars. As a second step we closed the AO loop ALFAs design, operation, and upgrade plans.

The ALFA Laser Guide Star: Operation and Results

The importance of laser guide stars to the practical usefulness ofadaptive optics cannot be understated, and yet there are veryfew working systems.This contribution discusses the current status of the ALFAlaser guide star, with regard to the particular difficulties encounteredwhile observing as well as both the expected performance and that so farachieved from scientific observations.A description is given of a number of experiments involving ALFA whichaim to determine the atmospheric turbulence and sodium layercharateristics, and demonstrate the possibility for tilt recovery fromlaser guide stars.

The ALFA Laser and Analysis Tools

The optimal performance of adaptive optics systems can only be maintainedif the wavefront reference is bright and compact.Experience has shown that both of these important criteria are remarkablydifficult to achieve with laser guide stars. This contribution gives an account of the methods by which ALFAattempts to reach them.First, the production of a high quality, high power laser beam isdescribed.However, this quality is unavoidably compromised along the path to thelaunch telescope.In order to rectify this, a new set of diagnostic tools which monitorthe quality of the out-going beam has been installed, and these arealso described.Lastly, we outline a number of possible modifications on which we areworking.If successful, these may allow a substantial improvement in the beamquality.

ALFA: first operational experience of the MPE/MPIA laser guide star system for adaptive optics

The sodium laser guide star adaptive optics system ALFA has been constructed at the Calar Alto 3.5m telescope. Following the first detection of the laser beacon on the wavefront sensor in 1997 the system is now being optimized for best performance. In this contribution we discuss the current status of the launch beam and the planned improvements and upgrades. We report on the performance level achieved when it is used with the adaptive optics system, and relate various aspects of our experience during operation of the system. We have begun to produce scientific result and mention two of these.

Adaptive Optics Integral Field Spectroscopy of the Young Stellar Objects in LkHα 225

Progress in understanding the embedded stars in LkHa225 has been hampered by their variability, making it hard to compare data taken at different times, and by the limited resolution of the available data, which cannot probe the small scales between the two stars. In an attempt to overcome these difficulties, we present new near-infrared data on this object taken using the ALFA adaptive optics system with the MPE 3D integral field spectrometer and the near-infrared camera Omega-Cass. The stars themselves have K-band spectra which are dominated by warm dust emission, analagous to class I-II for low mass YSOs, suggesting that the stars are in a phase where they are still accreting matter. On the other hand, the ridge of continuum emission between them is rather bluer, suggestive of extincted and/or scattered stellar light rather than direct dust emission. The compactness of the CO emission seen toward each star argues for accretion disks (which can also account for much of the K-band veiling) rather than a neutral wind. In contrast to other YSOs with CO emission, LkHa225 has no detectable Br_gamma emission. Additionally there is no H_2 detected on the northern star, although we do confirm that the strongest H_2 emission is on the southern star, where we find it is excited primarily by thermal mechanisms. A second knot of H_2 is observed to its northeast, with a velocity shift of -75kms and a higher fraction of non-thermal emission. This is discussed with reference to the H2O maser, the molecular outflow, and [S II] emission observed between the stars.Progress in understanding the embedded stars in LkHα 225 has been hampered by their variability, making it hard to compare data taken at different times, and by the limited resolution of the available data, which cannot probe the small scales between the two stars. In an attempt to overcome these difficulties, we present new near-infrared data on this object taken using the adaptive optics with a laser for astronomy adaptive optics system with the MPE 3D integral field spectrometer and the near-infrared camera Omega-Cass. The stars themselves have K-band spectra which are dominated by warm dust emission, analogous to classes I-II for low-mass young stellar objects (YSOs), suggesting that the stars are in a phase where they are still accreting matter. On the other hand, the ridge of continuum emission between them is rather bluer, suggestive of extinct and/or scattered stellar light rather than direct dust emission. The compactness of the CO emission seen toward each star argues for accretion disks (which can also account for much of the K-band veiling) rather than a neutral wind. In contrast to other YSOs with CO emission, LkHα 225 has no detectable Brγ emission. In addition, there is no H2 detected on the northern star, although we do confirm that the strongest H2 emission is on the southern star, where we find it is excited primarily by thermal mechanisms. A second knot of H2 is observed to its northeast, with a velocity shift of -75 km s-1 and a higher fraction of nonthermal emission. This is discussed with reference to the H2O maser, the molecular outflow, and [S II] emission observed between the stars.

ALFA: three years of experience in adaptive optics with a laser guide star

The Max-Planck institutes for astronomy and for extraterrestrial physics run a high order adaptive optics system with a laser guide star facility at the Calar Alto 3.5- m telescope in southern Spain. This system, called ALFA, saw first light in September 1996. Today, ALFA can compensate for atmospheric turbulences with natural guide stars as faint as 13.5th magnitude in R-band. ALFA recently succeeded in overcoming this limiting magnitude with the deployment of its laser guide star. This paper briefly reviews the ALFA project and its progress over the last 3 years. We further discuss the impact of sodium-layer laser guide stars on wavefront sensing and present results obtained with both kinds of guide stars.

Practical approach to modal basis selection and wavefront estimation

The MPIA/MPE adaptive optics with a laser guide star system ALFA works excellent with natural guide stars up to 13th magnitude in R-band. Using fainter natural guide stars or the extended laser guide star, ALFAs performance does not entirely satisfy our expectations. We describe our efforts in optimizing the wavefront estimation process. Starting with a detailed system analysis, this paper will show how to construct a modal basis set which efficiently uses Shack- Hartmann measurements while keeping a certain number of low order modes close to analytical basis sets like Zernikes or Karhunen-Loeve functions. We will also introduce various phase estimators (least squares, weighted least squares, maximum a posteriori) and show how these can be applied to the ALFA AO. A first test done at the Calar Alto 3.5-m-telescope will be discussed.