Calar Alto Observatory Research Articles

SAOLIM, Prototype of a Low-Cost System for Adaptive Optics with Lucky Imaging. Design and Performance

ABSTRACTA prototype of a low-cost adaptive optics (AO) system has been developed at the Instituto de Astrofisica de Andalucia (CSIC) and tested at the 2.2 m telescope of the Calar Alto Observatory. We present here the status of the project, which includes the image stabilization system and compensation of high-order wavefront aberrations with a membrane-deformable mirror. The image stabilization system consists of a magnet-driven tip-tilt mirror. The higher-order compensation system comprises a Shack-Hartmann sensor, a membrane-deformable mirror with 39 actuators, and the control computer, which allows operations up to 420 Hz in closed-loop mode. We have successfully closed the high-order AO loop on natural guide stars. An improvement of 4 times in terms of FWHM was achieved. The description and the results obtained on the sky are presented in this article.

Multi-band transit observations of the TrES-2b exoplanet

We present a new data set of transit observations of the TrES-2b exoplanet taken in spring 2009, using the 1.2m Oskar-Luhning telescope (OLT) of Hamburg Observatory and the 2.2m telescope at Calar Alto Observatory using BUSCA (Bonn University Simultaneous CAmera). Both the new OLT data, taken with the same instrumental setup as our data taken in 2008, as well as the simultaneously recorded multicolor BUSCA data confirm the low inclination values reported previously, and in fact suggest that the TrES-2b exoplanet has already passed the first inclination threshold (i_min,1 = 83.417) and is not eclipsing the full stellar surface any longer. Using the multi-band BUSCA data we demonstrate that the multicolor light curves can be consistently fitted with a given set of limb darkening coefficients without the need to adjust these coefficients, and further, we can demonstrate that wavelength dependent stellar radius changes must be small as expected from theory. Our new observations provide further evidence for a change of the orbit inclination of the transiting extrasolar planet TrES-2b reported previously. We examine in detail possible causes for this inclination change and argue that the observed change should be interpreted as nodal regression. While the assumption of an oblate host star requires an unreasonably large second harmonic coefficient, the existence of a third body in the form of an additional planet would provide a very natural explanation for the observed secular orbit change. Given the lack of clearly observed short-term variations of transit timing and our observed secular nodal regression rate, we predict a period between approximately 50 and 100 days for a putative perturbing planet of Jovian mass. Such an object should be detectable with present-day radial velocity (RV) techniques.

Recent GRBs Observed with the 1.23 m CAHA Telescope and the Status of Its Upgrade

We report on optical observations of Gamma‐Ray Bursts (GRBs) followed up by our collaboration with the 1.23 m telescope located at the Calar Alto observatory. The 1.23 m telescope is an old facility, currently undergoing upgrades to enable fully autonomous response to GRB alerts. We discuss the current status of the control system upgrade of the 1.23 m telescope. The upgrade is being done by our group based on the Remote Telescope System, 2nd Version (RTS2), which controls the available instruments and interacts with the EPICS database of Calar Alto. (Our group is called ARAE (Robotic Astronomy & High‐Energy Astrophysics) and is based on members of IAA (Instituto de Astrofísica de Andalucía). Currently the ARAE group is responsible to develop the BOOTES network of robotic telescopes (Jelínek et al. 2009).) Currently the telescope can run fully autonomously or under observer supervision using RTS2. The fast reaction response mode for GRB reaction (typically with response times below 3 minutes from the GRB onset) still needs some development and testing. The telescope is usually operated in legacy interactive mode, with periods of supervised autonomous runs under RTS2. We show the preliminary results of several GRBs followed up with observer intervention during the testing phase of the 1.23 m control software upgrade.

SEARCH FOR VERY LOW-MASS BROWN DWARFS AND FREE-FLOATING PLANETARY-MASS OBJECTS IN TAURUS

The number of low-mass brown dwarfs and even free floating planetary mass objects in young nearby star-forming regions and associations is continuously increasing, offering the possibility to study the low-mass end of the IMF in greater detail. In this paper, we present six new candidates for (very) low-mass objects in the Taurus star-forming region one of which was recently discovered in parallel by Luhman et al. (2009). The underlying data we use is part of a new database from a deep near-infrared survey at the Calar Alto observatory. The survey is more than four magnitudes deeper than the 2MASS survey and covers currently ~1.5 square degree. Complementary optical photometry from SDSS were available for roughly 1.0 square degree. After selection of the candidates using different color indices, additional photometry from Spitzer/IRAC was included in the analysis. In greater detail we focus on two very faint objects for which we obtained J-band spectra. Based on comparison with reference spectra we derive a spectral type of L2+/-0.5 for one object, making it the object with the latest spectral type in Taurus known today. From models we find the effective temperature to be 2080+/-140 K and the mass 5-15 Jupiter masses. For the second source the J-band spectrum does not provide a definite proof of the young, low-mass nature of the object as the expected steep water vapor absorption at 1.33 micron is not present in the data. We discuss the probability that this object might be a background giant or carbon star. If it were a young Taurus member, however, a comparison to theoretical models suggests that it lies close to or even below the deuterium burning limit (<13 Jupiter masses) as well. A first proper motion analysis for both objects shows that they are good candidates for being Taurus members.

PYRAMIR: Exploring the On-Sky Performance of the World’s First Near-Infrared Pyramid Wavefront Sensor

ABSTRACTThis paper presents the on-sky performance of the unmodulated infrared pyramid wavefront sensor PYRAMIR mounted on the ALFA adaptive optics system at the 3.5 m telescope of the Calar Alto Observatory. The performance of the system is compared with the performance of the Shack-Hartmann wavefront sensor of the ALFA system. We carried out a series of measurements to characterize the performance of PYRAMIR under open-loop tip-tilt compensation, and high-order closed-loop conditions, using stars of different magnitudes. We measured the tip-tilt jitter by following the centroid position of a stellar image on a fast series of frames. Additionally from the pyramid wavefront sensor data we could estimate the tip-tilt jitter in closed-loop. Under closed-loop conditions we also measured the long-exposure Strehl ratio. We compared the results of the wavefront sensor measurements with those of the Shack-Hartmann sensor on the same telescope, especially regarding the distribution of the error budged over the Karhunen-Loève modes, and the power spectral density. Our first finding is that we can indeed start up this nonmodulated pyramid system, even under bad seeing conditions. Under good conditions the Strehl ratio reaches ≥60% in K′ band. We found that the minimum signal-to-noise ratio (S/N) in each subaperture required to close the high-order loop is only 0.4. This is a surprisingly low number. To compare the performance to existing systems, we introduce the S/N per subaperture per loop cycle as a device-independent measure. Using this scheme, we find that the ratio between the low-order residuals and the high-order residuals in the case of PYRAMIR is lower than that of the Shack-Hartmann system, especially in the faint flux regime. This is an important finding because it means that the pyramid-based system removes the halo, i.e., light scattered by the atmosphere, around the target star better than a Shack-Hartmann sensor-based system. A comparison of the power spectral density of the PYRAMIR and the Shack-Hartmann sensor measurements, and a comparison of the noise propagation coefficients of PYRAMIR with theoretical predictions from the literature, confirm this superiority of the pyramid over the Shack-Hartmann sensor.

Mapping the starburst in blue compact dwarf galaxies

<i>Aims. <i/>By means of optical integral field spectroscopy observations, we aim to differentiate and characterize the starburst component in the blue compact dwarf (BCD) galaxy Mrk 1418. In particular we propose to study the stellar and ionized gas morphology, to investigate the ionization mechanism(s) acting in the interstellar medium, and to derive the physical parameters and abundances of the ionized gas.<i>Methods. <i/>Integral field spectroscopy observations of Mrk 1418 were carried out with the Potsdam multi-aperture spectrophotometer (PMAS) at the 3.5 m telescope at Calar Alto Observatory. The central 16″16″ (1.14 1.14 kpc<sup>2<sup/> at the distance of Mrk 1418) were mapped with a spatial sampling of 1″; we took data in the 3590-6996 Å spectral range, with a linear dispersion of 3.2 Å per pixel. The seeing was about 15. From these data we built maps of the most prominent emission lines, namely [O ii], H<i>β<i/>, [O iii], H<i>α<i/>, [N ii], and [S ii], as well as of several continuum bands, plus maps of the main line ratios: [O iii]/H<i>β<i/>, [N ii]/H<i>α<i/>, [S ii]/H<i>α<i/>, and H<i>α<i/>/H<i>β<i/>, and derived the physical parameters and gaseous metal abundances of the different star-forming regions detected in the field of view.<i>Results. <i/>Mrk 1418 shows a distorted morphology both in the continuum and in the ionized gas maps; the current star- formation episode is taking place in five knots, distributed around the nucleus of the galaxy. The interstellar medium surrounding these knots is photo-ionized by stars, with no clear evidence for other excitation mechanisms. The galaxy displays an inhomogeneous dust distribution, with the high H<i>α<i/>/H<i>β<i/> ratio in the central areas indicating a large amount of dust. The oxygen abundances derived for the individual star-forming knots are very similar, suggesting that the ionized interstellar medium is chemically homogeneous in O/H over spatial scales of hundreds of parsecs. This abundance (<i>Z<i/> <i>≈<i/> 0.4 from the empirical calibrations) places Mrk 1418 among the high metallicity BCD group.<i>Conclusions. <i/>These findings show the advantages of IFS when investigating the properties of such complex objects as BCDs, with an asymmetric star-forming component. Only a bidimensional mapping of their central regions allows approaching such questions as the star formation processes in BCDs, the star-forming history of the individual starburst knots, or the abundance gradients.

Dynamic masses for the close PG1159 binary SDSSJ212531.92-010745.9

The evolutionary scenarios which are commonly accepted for PG 1159 stars are mainly based on numerical simulations. These stellar evolution scenarios have to be tested and calibrated with real objects with known stellar parameters. One of the most crucial but also quite uncertain parameters is the stellar mass. PG 1159 stars have masses between 0.5 and 0.8 solar masses, as derived from asteroseismic and spectroscopic determinations. Such mass determinations are, however, themselves model-dependent. Moreover, asteroseismologically and spectroscopically determined masses deviate systematically for PG1159 stars by up to 10 %. SDSSJ212531.92-010745.9 is the first known PG1159 star in a close binary with a late main sequence companion allowing a dynamical mass determination. The system shows flux variations with a peak-to-peak amplitude of about 0.7 mag and a period of about 6.96 h. In August 2007, 13 spectra of SDSS J212531.92-010745.9 covering the full orbital phase range were taken at the TWIN 3.5m telescope at the Calar Alto Observatory (Almería, Spain). These confirm the typical PG1159 features seen in the SDSS discovery spectrum, together with the Balmer series of hydrogen in emission (plus other emission lines), interpreted as signature of the companion's irradiated side. A radial velocity curve was obtained for both components. Using co-added radial-velocity-corrected spectra, the spectral analysis of the PG 1159 star is being refined. The system's lightcurve, obtained during three seasons of photometry with the Göttingen 50 cm and Tübingen 80 cm telescopes, was fitted with both the NIGHTFALL and PHOEBE binary simulation programs. An accurate mass determination of the PG 1159 component from the radial velocity measurements requires to first derive the inclination, which requires light curve modelling and yields further constraints on radii, effective temperature and separation of the system's components. From the analysis of all data available so far, we present the possible mass range for the PG1159 component of SDSSJ212531.92-010745.9.

Activity evolution, outbursts, and splitting events of comet 73P/Schwassmann-Wachmann 3

Context. The split Jupiter family comet 73P/Schwassmann-Wachmann 3 was monitored between January 21 and May 25 2006, for 24 nights. Aims. The goals of the campaign were to characterize the two principal comet components (C and B) and the smaller component G during their approach to perihelion, and study differences and commonalities in their evolution to obtain insight into the nature of the nuclei (gas and dust). We aimed to assess the chemical homogeneity/heterogeneity of the different components, the presence of jets and other coma structures, the rotation axis, the long-term activity evolution, and the detection of new fragmentation events. Methods. Long-slit spectra and optical broadband images were acquired using the CAFOS instrument at the 2.2-m telescope at Calar Alto Observatory (CSIC-MPG). Data obtained in service mode consisted of spectra and Johnson R filter images. By observing for four nights close to perigee, the comet could be imaged in the Johnson UBVRI filters. When possible, we analyzed the radial profile of the dust brightness and we derived the dust and gas production rates, the dust reddening, and the N-S profiles of the CN, C2, and C3 column densities. The analysis of the morphological evolution of coma structures and the determination of the rotation axis is performed in a separate paper. Results. We found that components C and B behave differently during most of our observations. While component C did not show any sudden increase in dust productivity, as measured by Afρ, component B was characterized by a higher activity variation, exhibiting two outburst peaks and fragmentation events. Excluding outburst dates, component B always had lower dust productivity than component C. We also found differences in the behavior of the dust brightness radial profiles and in the dust colors. Differences in the dust colors are found also with respect to component G. In the spectral analysis, we found that both C and B components seem to be carbon-chain depleted, their compositions being almost the same. This indicates that the pre-split original intact nucleus probably had a homogenous composition. A two-dimensional color map of component B on May 13 shows relative color variations in the inner coma that can be interpreted qualitatively in terms of Mie theory as fragmentation of silicate dust particles emanating from the nucleus.

Integral field spectroscopy of two radio galaxies at z $\mathsf{\sim}$ 2.3

<i>Aims. <i/>In this article we study the morphology, kinematics and ionization properties of the giant ionized gas nebulae surrounding two high redshift radio galaxies, 4C 40.36 (<i>z<i/> = 2.27) and 4C 48.48 (<i>z<i/> = 2.34).<i>Methods. <i/>Integral field spectroscopy observations were taken using the PPAK bundle of the PMAS spectrograph, mounted on the 3.5 m on the Calar Alto Observatory, in order to cover a field-of-view of 64″ 72″ centered in each radio galaxy. The observations spanned 5 nights, using two different spectral resolutions (with <i>FWHM<i/> ~ 4 Å and ~8 Å respectively), covering the optical wavelength range from ~3700 Å to ~7100 Å, which corresponds to the rest-frame ultraviolet range from ~1100 Å to ~2000 Å. Various emission lines are detected within this wavelength range, including Ly<i>α<i/> (1216 Å), NV (1240 Å), CIV (1549 Å), HeII (1640 Å), OIII] (1663 Å) and CIII] (1909 Å). The dataset was used to derive the spatial distribution of the flux intensity of each of these lines and the gas kinematics. The properties of the emission lines in the nuclear regions were studied in detail.<i>Results. <i/>In agreement with previous studies, we find that both objects are embedded in a large ionized gas nebula, where Ly<i>α<i/> emission is extended across ~100 kpc or more. The CIV and HeII emission lines are also spatially extended. The nebulae are generally aligned with the radio axis, although we detect emission far from it. In 4C+48.48, there is a band of low Ly<i>α<i/>/CIV running perpendicular to the radio axis, at the location of the active nucleus. This feature might be the observational signature of an edge-on disk of neutral gas. The kinematics of both nebulae are inconsistent with stable rotation, although they are not inconsistent with infall or outflow.

The Night Sky at the Calar Alto Observatory II: The Sky at the Near-infrared

ABSTRACTWe present the characterization of additional properties of the night sky at the Calar Alto observatory, following the study started by Sánchez et al. in 2007. We focus here on the night-sky brightness at the near-infrared, the telescope seeing, and the fraction of useful time at the observatory. For this study we have collected a large data set comprising 7311 near-infrared images taken regularly along the last four years for the ALHAMBRA survey (J, H, and Ks bands), together with a more reduced data set of additional near-infrared images taken for the current study. In addition, we collected the information derived by the meteorological station at the observatory during the last 10 yar, together with the results from the cloud sensor for the last ∼2 yr. We analyze the dependency of the near-infrared night-sky brightness with the air mass and the seasons, studying its origins and proposing a zenithal correction. A strong correlation is found between the night-sky brightness in the Ks band and the air temperature, with a gradient of ∼-0.08 mag per 1°C. The typical (darkest) night-sky brightness in the J, H, and Ks-band are 15.95 mag (16.95 mag), 13.99 mag (14.98 mag), and 12.39 mag (13.55 mag), respectively. These values have been derived for the first time for this observatory, showing that Calar Alto is as dark in the near-infrared as most of the other astronomical sites in the world with which we could compare it. Only Mauna Kea is clearly darker in the Ks band, but not only compared to Calar Alto but to any other observatory in the world. The typical telescope seeing and its distribution were derived on the basis of the FWHM of the stars detected in the considered near-infrared images. This value, ∼1.0′′ when converted to the V band, is only slightly larger than the atmospheric seeing measured at the same time by the seeing monitor, ∼0.9′′. Therefore, the effects different from the atmosphere produce a reduced degradation on the telescope seeing, of the order of ∼10%. Finally we estimate the fraction of useful time based on the relative humidity, gust wind speed, and presence of clouds. This fraction, ∼72%, is very similar to the one derived in 2007, based on the fraction of time when the extinction monitor is working.

Cometary cryo-volcanism: Source regions and a model for the UT 2005 June 14 and other mini-outbursts on Comet 9P/Tempel 1

Data on the UT 2005 June 14 mini-outburst of Comet 9P/Tempel 1 taken from different viewpoints have been examined for morphological differences and parallax. The data were taken with the Hubble Space Telescope (HST), from the Deep Impact (DI) spacecraft, and from the Calar Alto Observatory, Spain. The mini-outburst source region was found to be located near 218 ± 6E, 6 ± 5N on the Deep Impact nucleus shape model. The mini-outburst occurred at ∼ 12 pm local solar time. The distribution of light in the mini-outburst is similar to that expected for an ejecta curtain. The method and software used to determine the surface location was checked using position angles of the impact ejecta plume as seen from DI and HST. The general region of impact was recovered and a downrange tilt of the ejecta curtain axis of 10.2 deg from the surface normal was found. We computed tracks of possible source regions for nine other mini-outbursts seen from DI. Five of these tracks converge on the 2005 June 14 event location. Three of the tracks converge at a second location near (60E, 20S), well separated from the first. Multiple mini-outbursts arise at each location either from a single source or from a few sources in close proximity. The mini-outbursts occur both at night and during the day indicating at most weak, if any, control by direct sunlight. The times of outburst are non-random with a preference for early afternoon, dusk and midnight. None of the mini-outbursts occurred near dawn. They occur at low latitudes (between ± 40 deg ) near the points where the principal axis of minimum moment of inertia cuts the surface. These regions are furthest from the center of figure and have the lowest effective surface gravity. We use these results to develop a conceptual model of the mini-outburst process and make comparisons with the theoretical calculations. We find that the tensile strength of the sub-surface material must be very low (e.g., ∼ 10 2 – 10 3 dynes cm −2 ) and, on the basis of features imaged on the western facet of the nucleus, suggest that inflation of the sub-surface may be occurring. Our model makes specific predictions about the kind of surface morphology that should result from mini-outburst activity. We show that one of the isolated rimless depressions and the close-packed depressions found in the Deep Impact images have the properties needed and identify them as possible sites of past and current mini-outburst activity.

Two new V-type asteroids in the outer Main Belt?

The identification of basaltic asteroids in the asteroid Main Belt and the description of their surface mineralogy is necessary to understand the diversity in the collection of basaltic meteorites. Basaltic asteroids can be identified from their visible reflectance spectra and are classified as V-type in the usual taxonomies. In this work, we report visible spectroscopic observations of two candidate V-type asteroids, (7472) Kumakiri and (10537) 1991 RY16, located in the outer Main Belt ( a > 2.85 UA ). These candidates have been previously identified by Roig and Gil-Hutton [2006. Selecting candidate V-type asteroids from the analysis of the Sloan Digital Sky Survey colors. Icarus 183, 411] using the Sloan Digital Sky Survey colors. The spectroscopic observations have been obtained at the Calar Alto Observatory, Spain, during observational runs in November and December 2006. The spectra of these two asteroids show the steep slope shortwards of 0.70 μ m and the deep absorption feature longwards of 0.75 μ m that are characteristic of V-type asteroids. However, the presence of a shallow but conspicuous absorption band around 0.65 μ m opens some questions about the actual mineralogy of these two asteroids. Such band has never been observed before in basaltic asteroids with the intensity we detected it. We discuss the possibility for this shallow absorption feature to be caused by the presence of chromium on the asteroid surface. Our results indicate that, together with (1459) Magnya, asteroids (7472) Kumakiri and (10537) 1991 RY16 may be the only traces of basaltic material found up to now in the outer Main Belt.

PYRAMIR: Calibration and Operation of a Pyramid Near-Infrared Wavefront Sensor

ABSTRACTThe concept of pyramid wavefront sensors (PWFSs) has been around about a decade by now. However there is still a great lack of characterizing measurements that allow the best operation of such a system under real life conditions at an astronomical telescope. In this article we, therefore, investigate the behavior and robustness of the pyramid infrared wavefront sensor PYRAMIR mounted at the 3.5 m telescope at the Calar Alto Observatory under the influence of different error sources both intrinsic to the sensor and arising in the preceding optical system. The intrinsic errors include diffraction effects on the pyramid edges and detector readout noise. The external imperfections consist of a Gaussian profile in the intensity distribution in the pupil plane during calibration, the effect of an optically resolved reference source, and noncommon-path aberrations. We investigated the effect of three differently sized reference sources on the calibration of the PWFS. For the noncommon-path aberrations the quality of the response of the system is quantified in terms of modal cross talk and aliasing. We investigate the special behavior of the system regarding tip-tilt control. From our measurements we derive the method to optimize the calibration procedure and the setup of a PWFS adaptive optics (AO) system. We also calculate the total wavefront error arising from aliasing, modal cross talk, measurement error, and fitting error in order to optimize the number of calibrated modes for on-sky operations. These measurements result in a prediction of on-sky performance for various conditions.

Intragroup diffuse light in compact groups of galaxies - II. HCG 15, 35 and 51

This continuing study of intragroup light in compact groups of galaxies aims to establish new constraints to models of formation and evolution of galaxy groups, specially of compact groups, which are a key part in the evolution of larger structures, such as clusters. In this paper we present three additional groups (HCG 15, 35 and 51) using deep wide-field B- and R-band images observed with the LAICA camera at the 3.5-m telescope at the Calar Alto observatory (CAHA). This instrument provides us with very stable flat-fielding, a mandatory condition for reliably measuring intragroup diffuse light. The images were analysed with the OV_WAV package, a wavelet technique that allows us to uncover the intragroup component in an unprecedented way. We have detected that 19,15 and 26 per cent of the total light of HCG 15, 35 and 51, respectively, are in the diffuse component, with colours that are compatible with old stellar populations and with mean surface brightness that can be as low as 28.4 B mag arcsec -2 . Dynamical masses, crossing times and mass-to-light ratios were recalculated using the new group parameters. Also tidal features were analysed using the wavelet technique.

The galaxy populations from the centers to the infall regions inz ≈ 0.25 clusters

We conducted a panoramic spectroscopic campaign with MOSCA at the Calar Alto observatory. We acquired spectra of more than 500 objects. Approximately 150 of these spectra were of galaxies that are members of six different clusters, which differ in intrinsic X-ray luminosity. The wavelength range allows us to quantify the star formation activity by using the OII and the Halpha lines. This activity is examined in terms of the large-scale environment expressed by the clustercentric distance of the galaxies as well as on local scales given by the spatial galaxy densities. A global suppression of star-formation is detected in the outskirts of clusters, at about 3Rvir. Galaxies with ongoing star-formation have similar activity, regardless of the environment. Therefore, the decline of the star-formation activity inside the investigated clusters is driven mainly by the significant change in the fraction of active versus passive populations. This suggests that the suppression of the star-formation activity occurs on short timescales. We detect a significant population of red star-forming galaxies whose colors are consistent with the red-sequence of passive galaxies. They appear to be in an intermediate evolutionary stage between active and passive types. Since a suppression of star-formation activity is measured at large clustercentric distances and low projected densities, purely cluster-specific phenomena cannot fully explain the observed trends. Therefore, as suggested by other studies, group preprocessing may play an important role in transforming galaxies before they enter into the cluster environment. Since models predict that a significant fraction of galaxies observed in the outskirts may have already transversed through the cluster center, the effects of ram-pressure stripping cannot be neglected. (ABRIDGED)

New active galactic nuclei detected in ROSAT All Sky Survey galaxies

Aims. The ROSAT ALL Sky Survey Bright Source Catalogue (RASS-BSC) has been correlated with the Catalogue of Principal Galaxies (PGC) to identify new extragalactic counterparts. 550 reliable optical counterparts have been detected. However there existed no optical spectra for about 200 Active Galactic Nuclei (AGN) candidates before the ROSAT ALL Sky Survey (RASS) was completed. Methods. We took optical spectra of 176 X-ray candidates and companions at ESO, Calar Alto observatory and McDonald observatory. When necessary we used a line profile decomposition to measure line fluxes, widths and centers to classify their type of activity. Results. We discuss the redshift-, linewidth-, as well as optical and X-ray luminosity distribution of our ROSAT selected sample. 139 galaxies of our 166 X-ray counterparts have been identified as AGN with 93 being Seyfert 1 galaxies (61%). Eighteen of them (20%) are Narrow Line Seyfert 1 galaxies. 34 X-ray candidates (21%) are LINERs and only eight candidates (5%) are Seyfert 2. The ratio of the number of Seyfert 1 galaxies to Seyfert 2 galaxies is about 11/1. Optical surveys result in ratios of 1/1.4. The high fraction of detected Seyfert 1 galaxies is explained by the sensitivity of the ROSAT to soft X-rays which are heavily absorbed in type 2 AGN. Two X-ray candidates are HII-galaxies and 25 candidates (15%) show no signs of spectral activity. The AGN in our RASS selected sample exhibit slightly higher optical luminosities ( M B = (-20.71 ± 1.75) mag) and similar X-ray luminosities (log($L_{\mathrm{X}}$ [ erg s -1 ] ) = 42.9 ± 1.7) compared to other AGN surveys. The H α line width distribution ( FWHM ) of our newly identified ROSAT AGN sample is similar to the line widths distribution based on SDSS AGN. However, our newly identified RASS AGN have rather reddish colors explaining why they have not been detected before in ultraviolet or blue excess surveys.

A new wavelet-based approach for the automated treatment of large sets of lunar occultation data

The introduction of infrared arrays for lunar occultations (LO) work and the improvement of predictions based on new deep IR catalogues have resulted in a large increase in the number of observable occultations. We provide the means for an automated reduction of large sets of LO data. This frees the user from the tedious task of estimating first-guess parameters for the fit of each LO lightcurve. At the end of the process, ready-made plots and statistics enable the user to identify sources which appear to be resolved or binary and to initiate their detailed interactive analysis. The pipeline is tailored to array data, including the extraction of the lightcurves from FITS cubes. Because of its robustness and efficiency, the wavelet transform has been chosen to compute the initial guess of the parameters of the lightcurve fit. We illustrate and discuss our automatic reduction pipeline by analyzing a large volume of novel occultation data recorded at Calar Alto Observatory. The automated pipeline package is available from the authors.

A study of photometric variations on the dwarf planet (136199) Eris

Context. Eris is the largest dwarf planet currently known in the solar system. Knowledge about its physical parameters is necessary to interpret the characteristics of these kinds of bodies. Aims. The goal of this work is to study Eris' short-term and long-term variability in order to determine the amplitude of the lightcurve, which can be linked to the degree of elongation of the body or to the degree of albedo heterogeneity on the surface of the dwarf planet. In addition, the rotation period can be determined. Methods. CCD photometric observations of the trans Neptunian object Eris in R band on 16 nights spanning two years were carried out using the 1.5 m telescope at Sierra Nevada Observatory (OSN), the 2.5 m Isaac Newton Telescope (INT) telescope at the Roque de los Muchachos Observatory, and the 2.2 m Telescope at Calar Alto Observatory. Results. The time-series analysis leads to indications of a short-term variability whose nature is not clear. It could be real or a result of data-reduction artifacts, such as contamination by close, faint-background stars. The most significant periodicities are 14 h or its double, but other possibilities cannot be ruled out, like a 32 h weaker peak in the periodogram. As for the amplitude of the lightcurve, we get a peak-to-peak variability of 0.01 ± 0.01 mag. The study of the long-term variability indicates that a long rotation period cannot be rejected, but the amplitude would be smaller than 0.06 mag. These results are compatible with a nearly spherical body that has a homogeneous surface.

The Discovery of Two Extremely Low Luminosity Milky Way Globular Clusters

We report the discovery of two extremely low luminosity globular clusters in the Milky Way Halo. These objects were detected in the Sloan Digital Sky Survey Data Release 5 and confirmed with deeper imaging at the Calar Alto Observatory. The clusters, Koposov 1 and Koposov 2, are located at $\sim 40-50$ kpc and appear to have old stellar populations and luminosities of only $M_V \sim -1$ mag. Their observed sizes of $\sim 3$ pc are well within the expected tidal limit of $\sim$10 pc at that distance. Together with Palomar 1, AM 4 and Whiting 1, these new clusters are the lowest luminosity globulars orbiting the Milky Way, with Koposov 2 the most extreme. Koposov 1 appears to lie close to distant branch of the Sagittarius stream. The half-mass relaxation times of Koposov 1 and 2 are only $\sim 70$ and $\sim 55$ Myr respectively (2 orders of magnitude shorter than the age of the stellar populations), so it would seem that they have undergone drastic mass segregation. Since they do not appear to be very concentrated, their evaporation timescales may be as low as $\sim 0.1 t_{\rm Hubble}$. These discoveries show that the structural parameter space of globular clusters in the Milky Way halo is not yet fully explored. They also add, through their short remaining survival times, significant direct evidence for a once much larger population of globular clusters.

A Preliminary Investigation of the Diffuse Interstellar Line at 8621 Å

ABSTRACTWe have obtained high‐resolution spectra at the Dominion Astrophysical Observatory, the Calar Alto Observatory, and the European Southern Observatory of hot stars in the near‐infrared region to study the diffuse interstellar band (DIB) at 8621 Å. Field stars as well as selected members of the Perseus spiral arm, the ρ Ophiuchi complex, and the Cygnus OB2 association were observed and the equivalent widths of the band measured, as well as interstellar K i absorption at 7699 Å. In total we measure the equivalent width of the 8621 band in 64 stars. In a series of figures we show the correlations of the DIB with reddening, polarization, K i, and strengths of other DIBs at 5780, 5797, and 6613 Å. The quality of the correlations are discussed by means of the Spearman rank correlation test. Good correlations are found with reddening and, among the other DIBs, 8621 correlates best with 5780. By examining the three special regions mentioned above we confirm the weakness of the 8621 DIB in the hot stars of the ρ Oph star‐forming region. In the Perseus spiral arm region we find that the DIBs are formed largely in foreground clouds but not in the gas that appears to have been blown away from the young massive stars of the Perseus arm. In Cyg OB2 we find that the equivalent widths of the 8621 line lie above the linear correlation of equivalent width with E(B - V) and their strength does not vary as strongly with reddening. The heavily reddened Cyg OB2 star 12 is the most extreme example of the relative insensitivity of the 8621 equivalent width to reddening in the Cygnus OB2 region.