Showing papers on "Photometry (optics) published in 2005"

The Photometric Performance and Calibration of the HST Advanced Camera For Surveys

Abstract: We present the photometric calibration of the HST Advanced Camera for Surveys (ACS). We give here an overview of the performance and calibration of the 2 CCD cameras, the Wide Field Channel (WFC) and the High Resolution Channel (HRC), and a description of the best techniques for reducing ACS CCD data. On-orbit observations of spectrophotometric standard stars have been used to revise the pre-launch estimate of the instrument response curves to best match predicted and observed count rates. Synthetic photometry has been used to determine zeropoints for all filters in 3 magnitude systems and to derive interstellar extinction values for the ACS photometric systems. Due to the CCD internal scattering of long wavelength photons, the width of the PSF increases significantly in the near-IR and the aperture correction for photometry with near-IR filters depends on the spectral energy distribution of the source. We provide encircled energy curves and a detailed recipe to correct for the latter effect. Transformations between the ACS photometric systems and the UBVRI and WFPC2 systems are presented. In general, two sets of transformations are available: 1 based on the observation of 2 star clusters; the other on synthetic photometry. We discuss the accuracy of these transformations and their sensitivity to details of the spectra being transformed. Initial signs of detector degradation due to the HST radiative environment are already visible. We discuss the impact on the data in terms of dark rate increase, charge transfer inefficiency, and hot pixel population.

Star counts in the Galaxy - Simulating from very deep to very shallow photometric surveys with the TRILEGAL code

Abstract: We describe TRILEGAL, a new populations synthesis code for simulating the stellar photometry of any Galaxy field. The code attempts to improve upon several technical aspects of star count models, by: dealing with very complete input libraries of evolutionary tracks; using a stellar spectral library to simulate the photometry in virtually any broad-band system; being very versatile allowing easy changes in the input libraries and in the description of all of its ingredients - like the star formation rate, age-metallicity relation, initial mass function, and geometry of Galaxy components. In a previous paper (Groenewegen et al. 2002, Paper I), the code was first applied to describe the very deep star counts of the CDFS stellar catalogue. Here, we briefly describe its initial calibration using EIS-deep and DMS star counts, which, as we show, are adequate samples to probe both the halo and the disc components of largest scale heights (oldest ages). We then present the changes in the calibration that were necessary to cope with some improvements in the model input data, and the use of more extensive photometry datasets: now the code is shown to successfully simulate also the relatively shallower 2MASS catalogue, which probes mostly the disc at intermediate ages, and the immediate solar neighbourhood as sampled by Hipparcos - in particular its absolute magnitude versus colour diagram -, which contains a somewhat larger fraction of younger stars than deeper surveys. Remarkably, the same model calibration can reproduce well the star counts in all the above-mentioned data sets, that span from the very deep magnitudes of CDFS (16 < R < 23) to the very shallow ones of Hipparcos (V < 8). Significant deviations (above 50 percent in number counts) are found just for fields close to the Galactic Center (since no bulge component was included) and Plane, and for a single set of South Galactic Pole data. The TRILEGAL code is ready to use for the variety of wide-angle surveys in the optical/infrared that will become available in the coming years.

Star counts in the Galaxy. Simulating from very deep to very shallow photometric surveys with the TRILEGAL code

Abstract: We describe TRILEGAL, a new populations synthesis code for simulating the stellar photometry of any Galaxy field. The code attempts to improve upon several technical aspects of star count models, by: dealing with very complete input libraries of evolutionary tracks; using a stellar spectral library to simulate the photometry in any broad-band system; being very versatile allowing easy changes in the input libraries and in the description of all of its ingredients -- like the SFR, AMR, IMF, and geometry of Galaxy components. In a previous paper (Groenewegen et al. 2002), the code was first applied to describe the very deep star counts of the CDFS stellar catalogue. Here, we briefly describe its initial calibration using EIS-deep and DMS star counts, which are adequate samples to probe both the halo and the disc components of largest scale heights (oldest ages). We then present the changes in the calibration that were necessary to cope with some improvements in the model input data, and the use of more extensive photometry datasets: the relatively shallower 2MASS catalogue, which probes mostly the disc at intermediate ages, and the immediate solar neighbourhood as sampled by Hipparcos, which contains a somewhat larger fraction of younger stars than deeper surveys. Remarkably, the same model calibration can reproduce well the star counts in all the above-mentioned data sets, that span from the very deep magnitudes of CDFS (16

Kinemetry: a generalisation of photometry to the higher moments of the line-of-sight velocity distribution

Abstract: We present a generalisation of surface photometry to the higher-order moments of the line-of-sight velocity distribution of galaxies observed with integral-field spectrographs. The generalisation follows the approach of surface photometry by determining the best fitting ellipses along which the profiles of the moments can be extracted and analysed by means of harmonic expansion. The assumption for the odd moments (e.g. mean velocity) is that the profile along an ellipse satisfies a simple cosine law. The assumption for the even moments (e.g velocity dispersion) is that the profile is constant, as it is used in surface photometry. We find that velocity profiles extracted along ellipses of early-type galaxies are well represented by the simple cosine law (with 2% accuracy), while possible deviations are carried in the fifth harmonic term which is sensitive to the existence of multiple kinematic components, and has some analogy to the shape parameter of photometry. We compare the properties of the kinematic and photometric ellipses and find that they are often very similar. Finally, we offer a characterisation of the main velocity structures based only on the kinemetric parameters which can be used to quantify the features in velocity maps (abridged).

Intergalactic stars in z∼ 0.25 galaxy clusters: systematic properties from stacking of Sloan Digital Sky Survey imaging data

Abstract: We analyse the spatial distribution and colour of the intracluster light (ICL) in 683 clusters of galaxies between z = 0.2 and 0.3, selected from ∼1500 deg 2 of the first data release of the Sloan Digital Sky Survey (SDSS-DR1). Surface photometry in the g-, r- and i-bands is conducted on stacked images of the clusters, after rescaling them to the same metric size and masking out resolved sources. We are able to trace the average surface brightness (SB) profile of the ICL out to 700 kpc, where it is less than 10 -4 of the mean surface brightness of the dark night sky. The ICL appears as a clear surface brightness excess with respect to an inner R 1/4 profile which characterizes the mean profile of the brightest cluster galaxies (BCGs). The surface brightness of the ICL ranges from 27.5 mag arcsec -2 at 100 kpc to ∼32 mag arcsec -2 at 700 kpc in the observed r-band. This corresponds to a value of SB in the range 26.5-31 in the rest-frame g-band. We find that, on average, the ICL contributes only a small fraction of the total optical emission in a cluster. Within a fixed metric aperture of 500 kpc, this fraction is 10.9 ± 5.0 per cent for our clusters. A further 21.9 ± 3.0 per cent is contributed on average by the BCG itself. The radial distribution of the ICL is more centrally concentrated than that of the cluster galaxies, but the colours of the two components are identical within the statistical uncertainties. In the mean the ICL is aligned with and more flattened than the BCG itself. This alignment is substantially stronger than that of the cluster light as a whole. We find the surface brightness of the ICL to correlate both with BCG luminosity and with cluster richness, while the fraction of the total light in the ICL is almost independent of these quantities. These results support the idea that the ICL is produced by stripping and disruption of galaxies as they pass through the central regions of clusters. Our measurements of the diffuse light also constrain the faint-end slope of the cluster luminosity function. Slopes a < -1.35 would imply more light from undetected galaxies than is observed in the diffuse component.

Standard Photometric Systems

Abstract: ▪ Abstract Standard star photometry dominated the latter half of the twentieth century reaching its zenith in the 1980s. It was introduced to take advantage of the high sensitivity and large dynamic range of photomultiplier tubes compared to photographic plates. As the quantum efficiency of photodetectors improved and the wavelength range extended further to the red, standard systems were modified and refined, and deviations from the original systems proliferated. The revolutionary shift to area detectors for all optical and IR observations forced further changes to standard systems, and the precision and accuracy of much broad- and intermediate-band photometry suffered until more suitable observational techniques and standard reduction procedures were adopted. But the biggest revolution occurred with the production of all-sky photometric surveys. Hipparcos/Tycho was space based, but most, like 2MASS, were ground-based, dedicated survey telescopes. It is very likely that in the future, rather than making ...

The Multiwavelength Survey by Yale-Chile (MUSYC): Survey Design and Deep Public UBVRIz' Images and Catalogs of the Extended Hubble Deep Field South

Abstract: We present UBVRIz' optical images of the 0.32 square degree Extended Hubble Deep Field South. This is one of four fields comprising the MUSYC survey, which is optimized for the study of galaxies at z=3, AGN demographics, and Galactic structure. We calculate corrected aperture photometry and its uncertainties and find through tests that these provide a significant improvement upon standard techniques. Our photometric catalog of 62968 objects is complete to a total magnitude of R_AB=25. We select z=3 Lyman break galaxy (LBG) candidates from their UVR colors and find a sky surface density of 1.4/sq. arcmin and an angular correlation function w(theta) = 2.3+-1.0 theta^{-0.8}. (Abridged)

A Jovian-mass Planet in Microlensing Event OGLE-2005-BLG-071

Abstract: We report the discovery of a several-Jupiter mass planetary companion to the primary lens star in microlensing event OGLE-2005-BLG-071. Precise (<1%) photometry at the peak of the event yields an extremely high signal-to-noise ratio detection of a deviation from the light curve expected from an isolated lens. The planetary character of this deviation is easily and unambiguously discernible from the gross features of the light curve. Detailed modeling yields a tightly-constrained planet-star mass ratio of q=m_p/M=0.0071+/-0.0003. This is the second robust detection of a planet with microlensing, demonstrating that the technique itself is viable and that planets are not rare in the systems probed by microlensing, which typically lie several kpc toward the Galactic center.

The XO Project: Searching for Transiting Extrasolar Planet Candidates

Abstract: The XO project’s first objective is to find hot Jupiters transiting bright stars, i.e. V < 12, by precision differential photometry. Two XO cameras have been operating since September 2003 on the 10,000-foot Haleakala summit on Maui. Each XO camera consists of a 200-mm f/1.8 lens coupled to a 1024x1024 pixel, thinned CCD operated by drift scanning. In its first year of routine operation, XO has observed 6.6% of the sky, within six 7 ◦ -wide strips scanned from 0 ◦ to +63 ◦ of declination and centered at RA=0, 4, 8, 12, 16, and 20 hours. Autonomously operating, XO records 1 billion pixels per clear night, calibrates them photometrically and astrometrically, performs aperture photometry, archives the pixel data and transmits the photometric data to STScI for further analysis. From the first year of operation, the resulting database consists of photometry of �100,000 stars at more than 1000 epochs per star with differential photometric precision better than 1% per epoch. Analysis of the light curves of those stars produces transiting-planet candidates requiring detailed follow up, described elsewhere, culminating in spectroscopy to measure radial-velocity variation in order to differentiate genuine planets from the more numerous impostors, primarily eclipsing binary and multiple stars.

A Minor-Axis Surface Brightness Profile for M31

Abstract: We use data from the Isaac Newton Telescope Wide-Field Camera survey of M31 to determine the surface brightness profile of M31 along the southeast minor axis. We combine surface photometry and faint red giant branch star counts to trace the profile from the innermost regions out to a projected radius of 4° (≈55 kpc), where μV ~ 32 mag arcsec-2; this is the first time the M31 minor-axis profile has been mapped over such a large radial distance using a single data set. We confirm the finding by Pritchet & van den Bergh that the minor-axis profile can be described by a single de Vaucouleurs law out to a projected radius of 14 or ≈20 kpc. Beyond this, the surface brightness profile flattens considerably and is consistent with either a power law of index about -2.3 or an exponential of scale length 14 kpc. The fraction of the total M31 luminosity contained in this component is ≈2.5%. While it is tempting to associate this outer component with a true Population II halo in M31, we find that the mean color of the stellar population remains roughly constant at V - i ≈ 1.6 from 05 to 35 along the minor axis. This result suggests that the same metal-rich stellar population dominates both structural components.

The XO Project: Searching for Transiting Extra-solar Planet Candidates

Abstract: The XO project's first objective is to find hot Jupiters transiting bright stars, i.e. V < 12, by precision differential photometry. Two XO cameras have been operating since September 2003 on the 10,000-foot Haleakala summit on Maui. Each XO camera consists of a 200-mm f/1.8 lens coupled to a 1024x1024 pixel, thinned CCD operated by drift scanning. In its first year of routine operation, XO has observed 6.6% of the sky, within six 7 deg-wide strips scanned from 0 deg to +63 deg of declination and centered at RA=0, 4, 8, 12, 16, and 20 hours. Autonomously operating, XO records 1 billion pixels per clear night, calibrates them photometrically and astrometrically, performs aperture photometry, archives the pixel data and transmits the photometric data to STScI for further analysis. From the first year of operation, the resulting database consists of photometry of 100,000 stars at more than 1000 epochs per star with differential photometric precision better than 1% per epoch. Analysis of the light curves of those stars produces transiting-planet candidates requiring detailed follow up, described elsewhere, culminating in spectroscopy to measure radial-velocity variation in order to differentiate genuine planets from the more numerous impostors, primarily eclipsing binary and multiple stars.

NGC 300: An Extremely Faint, Outer Stellar Disk Observed to 10 Scale Lengths

Abstract: We have used the Gemini Multi-Object Spectrograph (GMOS) on the Gemini South 8 m telescope in exceptional conditions (06 FWHM seeing) to observe the outer stellar disk of the Sculptor Group galaxy NGC 300 at two locations. At our point-source detection threshold of r' = 27.0 (3 σ) mag, we trace the stellar disk out to a radius of 24', or 2.2R25, where R25 is the 25 mag arcsec-2 isophotal radius. This corresponds to about 10 scale lengths in this low-luminosity spiral galaxy (MB = -18.6), or about 14.4 kpc at a Cepheid distance of 2.0 ± 0.07 Mpc. The background galaxy counts are derived in the outermost field, and these are within 10% of the mean survey counts from both Hubble Deep Fields. The luminosity profile is well described by a nucleus plus a simple exponential profile out to 10 optical scale lengths. We reach an effective surface brightness of μ = 30.5 mag arcsec-2 (2 σ) at 55% completeness, which doubles the known radial extent of the optical disk. These levels are exceedingly faint in the sense that the equivalent surface brightness in B or V is about 32 mag arcsec-2. We find no evidence for truncation of the stellar disk. Only star counts can be used to reliably trace the disk to such faint levels, since surface photometry is ultimately limited by nonstellar sources of radiation. In the Appendix, we derive the expected surface brightness of one such source: dust scattering of starlight in the outer disk.

EDisCS - the ESO distant cluster survey. Sample definition and optical photometry

Abstract: We present the ESO Distant Cluster Survey (EDisCS), a survey of 20 fields containing distant galaxy clusters with redshifts ranging from 0.4 to almost 1.0. Candidate clusters were chosen from among the brightest objects identified in the Las Campanas Distant Cluster Survey, half with estimated redshift z est ∼ 0.5 and half with z est ∼ 0.8. They were confirmed by identifying red sequences in moderately deep two colour data from VLT/FORS2. For confirmed candidates we have assembled deep three-band optical photometry using VLT/FORS2, deep near-infrared photometry in one or two bands using NTT/SOFI, deep optical spectroscopy using VLT/FORS2, wide field imaging in two or three bands using the ESO Wide Field Imager, and HST/ACS mosaic images for 10 of the most distant clusters. This first paper presents our sample and the VLT photometry we have obtained. We present images, colour-magnitude diagrams and richness estimates for our clusters, as well as giving redshifts and positions for the brightest cluster members. Subsequent papers will present our infrared photometry, spectroscopy, HST and wide-field imaging, as well as a wealth of further analysis and science results. Our reduced data become publicly available as these papers are accepted.

Near-infrared properties of i-drop galaxies in the Hubble Ultra Deep Field

Abstract: We analyse near-infrared Hubble Space Telescope (HST)/Near-Infrared Camera and Multi-Object Spectrometer F110W (J) and F160W (H) band photometry of a sample of 27 i′-drop candidate z≃ 6 galaxies in the central region of the HST/Advanced Camera for Surveys Ultra Deep Field. The infrared colours of the 20 objects not affected by near neighbours are consistent with a high-redshift interpretation. This suggests that the low-redshift contamination of this i′-drop sample is smaller than that observed at brighter magnitudes, where values of 10–40 per cent have been reported. The J–H colours are consistent with a slope flat in fν(fλ∝λ−2), as would be expected for an unreddened starburst. However, there is evidence for a marginally bluer spectral slope (fλ∝λ−2.2), which is perhaps indicative of an extremely young starburst (∼10 Myr old) or a top heavy initial mass function and little dust. The low levels of contamination, median photometric redshift of z∼ 6.0 and blue spectral slope, inferred using the near-infrared data, support the validity of the assumptions in our earlier work in estimating the star formation rates, and that the majority of the i-drop candidates galaxies lie at z∼ 6.

Homogeneous Photometry IV: On the Standard Sequence in the Globular Cluster NGC2419

Abstract: A new analysis of CCD-based BVRI broad-band photometry for the globular cluster NGC2419 is presented, based on 340 CCD images either donated by colleagues or retrieved from public archives. The calibrated results have been made available through my Web site. I compare the results of my analysis with those of an independent analysis of a subset of these data by Saha et al. (2005, PASP, 117, 37), who have found a color-dependent discrepancy of up to 0.05 mag between their I-band photometry and mine for stars in this cluster. I conclude that a major part of the discrepancy appears to be associated with small (a few hundredths of a second) shutter-timing errors in the MiniMos camera on the WIYN 3.5-m telescope. Smaller contributions to the anomaly likely come from (a) a color-scale error with a maximum amplitude of ~ +/-0.02 mag in my secondary standard list as of September 2004; and (b) statistical effects arising from the previous study's use of a relatively small number of standard-star observations to determine a comparatively large number of fitting parameters in the photometric transformations.

Homogeneous Photometry. IV. On the Standard Sequence in the Globular Cluster NGC 2419

Abstract: I present a new analysis of CCD-based BVRI broadband photometry for the globular cluster NGC 2419, based on 340 CCD images either donated by colleagues or retrieved from public archives. The calibrated results have been made available through my Web site. I compare the results of my analysis with those of an independent analysis of a subset of these data by Saha et al. (2005, PASP, 117, 37), who have found a color-dependent discrepancy of up to 0.05 mag between their I-band photometry and mine for stars in this cluster. I conclude that a major part of the discrepancy appears to be associated with small shutter timing errors (a few hundredths of a second) in the Mini-Mosaic (MIMO) camera on the WIYN 3.5 m telescope. Smaller contributions to the anomaly likely come from (1) a color-scale error with a maximum amplitude of ~±0.02 mag in my secondary standard list as of 2004 September, and (2) statistical effects arising from the previous study's use of a relatively small number of standard-star observations to determine a comparatively large number of fitting parameters in the photometric transformations.

The Evolution of the Optical and Near-Infrared Galaxy Luminosity Functions and Luminosity Densities to z ~ 2

Abstract: Using Hubble Space Telescope and ground-based U through Ks photometry from the Great Observatories Origins Deep Survey, we measure the evolution of the luminosity function and luminosity density in the rest-frame optical (U, B, and R) to z ~ 2, bridging the poorly explored "redshift desert" between z ~ 1 and ~2. We also use deep near-infrared observations to measure the evolution in the rest-frame J band to z ~ 1. Compared to local measurements from the SDSS, we find a brightening of the characteristic magnitude, M*, by ~2.1, ~0.8, and ~0.7 mag between z ~ 0.1 and ~1.9, in U, B, and R, respectively. The evolution of M* in the J band is in the opposite sense, showing a dimming between redshifts z ~ 0.4 and 0.9. This is consistent with a scenario in which the mean star formation rate in galaxies was higher in the past, while the mean stellar mass was lower, in qualitative agreement with hierarchical galaxy formation models. We find that the shape of the luminosity function is strongly dependent on spectral type and that there is strong evolution with redshift in the relative contribution from the different spectral types to the luminosity density. We find good agreement with previous measurements, supporting an increase in the B-band luminosity density by a factor of ~2 between the local value and z ~ 1, and little evolution between z ~ 1 and ~2. We provide estimates of the uncertainty in our luminosity density measurements due to cosmic variance. We find good agreement in the luminosity function derived from an R-selected and a Ks-selected sample at z ~ 1, suggesting that optically selected surveys of similar depth (R 24) are not missing a significant fraction of objects at this redshift relative to a near-infrared-selected sample. We compare the rest-frame B-band luminosity functions from z = 0 to 2 with the predictions of a semianalytic hierarchical model of galaxy formation and find qualitatively good agreement. In particular, the model predicts at least as many optically luminous galaxies at z ~ 1-2 as are implied by our observations.

Metal Abundances of KISS Galaxies. IV. Galaxian Luminosity-Metallicity Relations in the Optical and Near-IR

Abstract: We explore the galaxian luminosity-metallicity (L-Z) relationship in both the optical and the near-IR using a combination of optical photometric and spectroscopic observations from the KPNO International Spectroscopic Survey (KISS) and near-infrared photometry from the Two-micron All Sky Survey (2MASS). We supplement the 2MASS data with our own NIR photometry for a small number of lower-luminosity ELGs that are under-represented in the 2MASS database. Our B-band L-Z relationship includes 765 star-forming KISS galaxies with coarse abundance estimates from our follow-up spectra, while the correlation with KISS and 2MASS yields a total of 420 galaxies in our J-band L-Z relationship. We explore the effect that changing the correlation between the strong-line abundance diagnostic R_23 and metallicity has on the derived L-Z relation. We find that the slope of the L-Z relationship decreases as the wavelength of the luminosity bandpass increases. We interpret this as being, at least in part, an effect of internal absorption in the host galaxy. Furthermore, the dispersion in the L-Z relation decreases for the NIR bands, suggesting that variations in internal absorption contribute significantly to the observed scatter. We propose that our NIR L-Z relations are more fundamental than the B-band relation, since they are largely free of absorption effects and the NIR luminosities are more directly related to the stellar mass of the galaxy than are the optical luminosities.

Metal Abundances of KISS Galaxies. IV. Galaxian Luminosity-Metallicity Relations in the Optical and Near-Infrared

Abstract: We explore the galaxian luminosity-metallicity (L-Z) relationship in both the optical and the near-infrared (NIR) using a combination of optical photometric and spectroscopic observations from the Kitt Peak National Observatory (KPNO) International Spectroscopic Survey (KISS) and NIR photometry from the Two Micron All Sky Survey (2MASS). We supplement the 2MASS data with our own NIR photometry for a small number of lower luminosity emission-line galaxies (ELGs) that are underrepresented in the 2MASS database. Our B-band L-Z relationship includes 765 star-forming KISS galaxies with coarse abundance estimates from our follow-up spectra, while the correlation with KISS and 2MASS yields a total of 420 galaxies in our J-band L-Z relationship. We explore the effect that changing the correlation between the strong-line abundance diagnostic R23 and metallicity has on the derived L-Z relation. We find that the slope of the L-Z relationship decreases as the wavelength of the luminosity bandpass increases. We interpret this as being, at least in part, an effect of internal absorption in the host galaxy. Furthermore, the dispersion in the L-Z relation decreases for the NIR bands, suggesting that variations in internal absorption contribute significantly to the observed scatter. We propose that our NIR L-Z relations are more fundamental than the B-band relation, since they are largely free of absorption effects and the NIR luminosities are more directly related to the stellar mass of the galaxy than are the optical luminosities.

Halpha surface photometry of galaxies in nearby clusters. V: the survey completion

Abstract: We present the Halpha imaging observations of 273 late-type galaxies in the nearby rich galaxy clusters Virgo, A1367, Coma, Cancer, Hercules and in the Great Wall, carried out primarily with the 2.1m telescope of the San Pedro Martir Observatory (SPM) and with the ESO/3.6m telescope. We derived the Halpha+[NII] fluxes and equivalent widths. The Halpha survey reached completion for an optically selected sample of nearby galaxies in and outside rich clusters. Taking advantage of the completeness of the data set, the dependence of Halpha properties on the Hubble type was determined for late-type galaxies in the Virgo cluster. Differences in the gaseous content partly account for the large scatter of the Halpha E.W. within each Hubble-type class. We studied the radial distributions of the Halpha E.W. around Coma+A1367 and the Virgo clusters in two luminosity bins. Luminous galaxies show a decrease in their average Halpha E.W. in the inner 1 virial radius, while low-luminosity galaxies do not show this trend.

Night Sky Photometry with Sky Quality Meter

Abstract: Sky Quality Meter, a low cost and pocket size night sky brightness photometer, opens to the general public the possibility to quantify the quality of the night sky. Expecting a large difiusion of measurements taken with this instrument, I tested and characterized it. I analyzed with synthetic photometry and laboratory measurements the relationship between the SQM photometrical system and the main systems used in light pollution studies. I evaluated the conversion factors to Johnson’s B and V bands, CIE photopic and CIE scotopic responses for typical spectra and the spectral mismatch correction factors when speciflc fllters are added. Subject headings: light pollution { night sky brightness { photometry { instruments { calibration

An Atlas of Spectrophotometric Landolt Standard Stars

Abstract: We present CCD observations of 102 Landolt standard stars obtained with the Ritchey‐Chretien spectrograph on the Cerro Tololo Inter‐American Observatory 1.5 m telescope. Using stellar atmosphere models, we have extended the flux points to our six spectrophotometric secondary standards, in both the blue and the red, allowing us to produce flux‐calibrated spectra that span a wavelength range from 3050 A to 1.1 μm. Mean differences between UBVRI spectrophotometry computed using Bessell’s standard passbands and Landolt’s published photometry were determined to be 1% or less. Observers in both hemispheres will find these spectra useful for flux‐calibrating spectra, and through the use of accurately constructed instrumental passbands, will be able to compute accurate corrections to bring instrumental magnitudes to any desired standard photometric system (S‐corrections). In addition, by combining empirical and modeled spectra of the Sun, Sirius, and Vega, we calculate and compare synthetic photometry to...

Predicting accurate stellar angular diameters by the near-infrared surface brightness technique

Abstract: I report on the capabilities of the near-infrared (near-IR) surface brightness technique to predict reliable stellar angular diameters as accurate as2 per cent using standard broad-band Johnson photometry in the colour range −0.1 (V − K )O 3.7 including stars of A, F, G, K spectral type. This empirical approach is fast to apply and leads to estimated photometric diameters in very good agreement with recent high-precision interferometric diameter measurements available for non-variable dwarfs and giants, as well as for Cepheid variables. Then I compare semi-empirical diameters predicted by model-dependent photometric and spectrophotometric (SP) methods with near-IR surface brightness diameters adopted as empirical reference calibrators. The overall agreement between all these methods is within approximately ±5 per cent, confirming previous works. However, on the same scale of accuracy, there is also evidence for systematic shifts presumably as a result of an incorrect representation of the stellar effective temperature in the model-dependent results. I also compare measurements of spectroscopic radii with near-IR surface brightness radii of Cepheids with known distances. Spectroscopic radii are found to be affected by a scatter as significant as 9 per cent, which is at least three times greater than the formal error currently claimed by the spectroscopic technique. In contrast, pulsation radii predicted by the period‐radius (PR) relation according to the Cepheid period result are significantly less dispersed, indicating a quite small scatter as a result of the finite width of the Cepheid instability strip, as expected from pulsation theory. The resulting low level of noise strongly confirms our previous claims that the pulsation parallaxes are the most accurate empirical distances presently available for Galactic and extragalactic Cepheids. Ke yw ords: stars: fundamental parameters ‐ Cepheids ‐ infrared: stars.

Pulsations of the Oe star ζ ophiuchi from MOST satellite photometry and ground-based spectroscopy

Abstract: Twenty-four days of highly precise Microvariability and Oscillations of Stars (MOST) satellite photometry obtained in mid-2004 of the rapidly rotating O9.5 V star ζ Oph have yielded at least a dozen significant oscillation frequencies between 1 and 10 cycles day-1, clearly indicating its relationship to β Cephei variables. Eight periods were found in He I λ4922 and Hβ line profile variations (LPV) of which six coincide with those from the MOST photometry. This unique photometric and spectroscopic detection of radial and nonradial pulsations leads to a plausible model in which high l-modes are excited when their frequencies in the corotating frame are similar to those of low-order radial modes. We propose that the dominant photometric 4.6 hr period (f1) corresponds to a radial first overtone excited by the κ-mechanism associated with the Fe opacity bump. No unambiguous rotational period can be identified in either the light curve or the LPV.

Stellar photometry and astrometry with discrete point spread functions

Abstract: The key features of the MATPHOT algorithm for precise and accurate stellar photometry and astrometry using discrete point spread functions (PSFs) are described. A discrete PSF is a sampled version of a continuous PSF, which describes the two-dimensional probability distribution of photons from a point source (star) just above the detector. The shape information about the photon scattering pattern of a discrete PSF is typically encoded using a numerical table (matrix) or an FITS (Flexible Image Transport System) image file. Discrete PSFs are shifted within an observational model using a 21-pixel-wide damped sinc function, and position-partial derivatives are computed using a five-point numerical differentiation formula. Precise and accurate stellar photometry and astrometry are achieved with undersampled CCD (charge-coupled device) observations by using supersampled discrete PSFs that are sampled two, three or more times more finely than the observational data. The precision and accuracy of the MATPHOT algorithm is demonstrated by using the C-language MPD code to analyse simulated CCD stellar observations; measured performance is compared with a theoretical performance model. Detailed analysis of simulated Next Generation Space Telescope observations demonstrate that millipixel relative astrometry and mmag photometric precision is achievable with complicated space-based discrete PSFs. Ke yw ords: methods: analytical ‐ methods: numerical ‐ methods: statistical ‐ techniques: image processing ‐ techniques photometric ‐ astrometry.

The Orion Nebula in the Mid-Infrared

Abstract: We present two wide-field (≈5' × 35), diffraction-limited (λ/D 05 at 10 μm), broadband 10 and 20 μm images of the Orion Nebula, plus six 7–13 μm narrowband (λ/Δλ 1) images of the BN/KL complex taken at the 3.8 m UKIRT telescope with the MPIA MAX camera. The wide-field images, centered on the Trapezium and BN/KL regions, are mosaics of 35'' × 35'' frames obtained with standard chopping and nodding techniques and reconstructed using a new restoration method developed for this project. They show the filamentary structure of the dust emission from the walls of the H II region and reveal a new remarkable group of arclike structures ≈1' to the south of the Trapezium. The morphology of the Ney-Allen Nebula, produced by wind-wind interaction in the vicinity of the Trapezium stars, suggests a complex kinematical structure at the center of the cluster. We find indications that one of the most massive members of the cluster, the B0.5 V star θ1 Ori D, is surrounded by a photoevaporated circumstellar disk. Among the four historic Trapezium OB stars, this is the only one without a binary companion, suggesting that stellar multiplicity and the presence of massive circumstellar disks may be mutually exclusive. In what concerns the BN/KL complex, we find evidence for extended optically thin silicate emission on top of the deep 10 μm absorption feature. Assuming a simple two-component model, we map with 05 spatial resolution the foreground optical depth, color temperature, and mid-IR luminosity of the embedded sources. We resolve a conspicuous point source at the location of the IRc2-A knot, approximately 05 north of the deeply embedded H II region I. We analyze the spectral profile of the 10 μm silicate absorption feature and find indication for grain crystallization in the harsh nebular environment. In the OMC-1 South region, we detect several point sources and discuss their association with the mass-loss phenomenology observed at optical and millimeter wavelengths. Finally, we list the position and photometry of 177 point sources, the large majority of which are detected for the first time in the mid-IR. Twenty-two of them lack a counterpart at shorter wavelengths and are therefore candidates for deeply embedded protostars. The comparison of photometric data obtained at two different epochs reveals that source variability at 10 μm is present up to a level of ≈1 mag on a timescale of ~2 yr. With the possible exception of a pair of OB stars, all point sources detected at shorter wavelengths display 10 μm emission well above the photospheric level, which we attribute to disk circumstellar emission. The recent model of Robberto et al. provides the simplest explanation for the observed mid-IR excess.

New Colour Transformations for the Sloan Photometry, and Revised Metallicity Calibration and Equations for Photometric Parallax Estimation

Abstract: We evaluated new colour transformations for the Sloan photometry by 224 standards and used them to revise both the equations for photometric parallax estimation and metallicity calibration cited by Karaali et al. (2003). This process improves the metallicity and absolute magnitude estimations by [Fe/H] ≤ 0.3 dex and MHg' ≥ 0.1 mag respectively. There is a high correlation for metallicities and absolute magnitudes derived for two systems, UBV and Sloan, by means of the revised calibrations.

Photometry of the Type Ia Supernovae 1999cc, 1999cl, and 2000cf

Abstract: We present previously unpublished BVRI photometry of the Type Ia supernovae 1999cc and 2000cf along with revised photometry of SN 1999cl. We confirm that SN 1999cl is reddened by highly non-standard dust, with R_V = 1.55 +/- 0.08. Excepting two quasar-lensing galaxies whose low values of R_V are controversial, this is the only known object with a published value of R_V less than 2.0. SNe 1999cl and 2000cf have near-infrared absolute magnitudes at maximum in good agreement with other Type Ia SNe of mid-range decline rates.

The Ultraviolet Luminosity Function of GALEX Galaxies at Photometric Redshifts between 0.07 and 0.25

Abstract: We present measurements of the UV galaxy luminosity function and the evolution of luminosity density from Galaxy Evolution Explorer (GALEX) observations matched to the Sloan Digital Sky Survey (SDSS). We analyze galaxies in the Medium Imaging Survey overlapping the SDSS First Data Release, with a total coverage of 44 deg2. Using the combined GALEX + SDSS photometry, we compute photometric redshifts and study the luminosity function in three redshift shells between z = 0.07 and 0.25. The Schechter function fits indicate that the faint-end slope α is consistent with -1.1 at all redshifts, but the characteristic UV luminosity M* brightens by 0.2 mag from z = 0.07 to 0.25. In the lowest redshift bin, early- and late-type galaxies are studied separately, and we confirm that red galaxies tend to be brighter and have a shallower slope α than blue ones. The derived luminosity densities are consistent with other GALEX results based on a local spectroscopic sample from the Two-Degree Field, and the evolution follows the trend reported by deeper studies.

GALEX OBSERVATIONS OF AN ENERGETIC ULTRAVIOLET FLARE ON THE dM4e STAR GJ 3685A

Abstract: The Galaxy Evolution Explorer (GALEX) satellite has obtained high time resolution ultraviolet photometry during a large flare on the M4 dwarf star GJ 3685A. Simultaneous Near-ultraviolet (NUV, 1750-2800 A) and Far-ultraviolet (FUV, 1350-1750 A) time-tagged photometry with time resolution better than 0.1 s shows that the overall brightness in the FUV band increased by a factor of 1000 in 200 s. Under the assumption that the NUV emission is mostly due to a stellar continuum, and that the FUV flux is shared equally between emission lines and continuum, there is evidence for two distinct flare components for this event. The first flare type is characterized by an exponential increase in flux with little or no increase in temperature. The other involves rapid increases in both temperature and flux. While the decay time for the first flare component may be several hours, the second flare event decayed over less than 1 minute, suggesting that there was little or no confinement of the heated plasma.