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Spectral Templates - Observations and Data Reduction

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This page contains an abbreviated description of the observations and data reduction. More details can be found in "The Gemini Spectral Library of Near-IR Late-Type Stellar Templates and Its Application for Velocity Dispersion Measurements". Winge, Cláudia; Riffel, Rogemar; Storchi-Bergmann, Thaisa, 2009, ApJS, 185, 186. Users are particularly referred to that paper for examples in the usage of the templates.

The GNIRS sample:

The observations were done using the Integral Field Unit (IFU) in the Gemini Near-Infrared Spectrograph (GNIRS) at Gemini South, with the grating 110.5 l/mm, yielding a resolving power of R~6000 (FWHM=3.4Å at 2.293μm).  The list of observations is given below, where for each spectral setting we present the date(s), the total exposure time on source and the hot star used to correct for the telluric lines. All GNIRS data were obtained as part of programme GS-2006B-DD-3.

Note that the IFU mode is no longer available with GNIRS at Gemini North.

Star "Blue" setting "Red" Setting
UT date Exp time (s) Tell. std UT date Exp time (s) Tell. std
HD20038 20061013
HR607 20060912
HD209750 20061020 108 HR7950
HD6461 20061010 560 HR100 20060905
HD173764 20061012 180 HR7136 20060904
HD36079 20070104 72 HR2020 20060914 72 HR2020
HD1737 20061008 180 HR100 20060914 180 HR100
HD213789 20060904 360 HR8959
HD212320 20061006 480 HR8959 20060904 480 HR8959
HD213009 20060903 90 HR8959
HD35369 20061015 180 HR2020 20061005 180 HR2020
HD64606 20070104 960 HR3314 20070102 960 HR3314
HD224533 20070107 120 HR8959 20060914 120 HR8959
HD4188 20061012 160 HR100 20061005 120 HR100
HD206067 20061006 180 HR8959 20060831 180 HR7950
HD34642 20061017 180 HR2020 20060912 180 HR2020
HD198700 20060904
HD218594 20061020 120 HR8959 20060903 36 HR8959
HD26965 20061212 180 HR2020 20060913
HD39425 20061212 36 HR2020 20061001 36 HR2020
HD38392 20061017 600 HR2020 20061001 360 HR2020
HD4730 20061008 240 HR100 20060903 96 HR100
HD191408 20060905
HD9138 20061019 144 HR607 20060910 90 HR607
HD720 20061012 144 HR100 20060910 144 HR100
HD32440 20070106 320 HR705 20070104 320 HR705
HD63425B 20061212 240 HR2672 20061212 240 HR2672
HD113538 20070104 1000 HR4933 20070106 1000 HR4933
HD2490 20061015 144 HR100 20060911 144 HR100
HD112300 20070116 30 HR5107

Each group of observations included a science target, one star (two standards were provided, giving better airmass match if observed before or after the target, or if observed before or after the target transited, but only one observed), a set of calibrations comprising three arcs and a set of 10 GCAL flats. Calibrations (arcs and flats) were usually observed right after the science target, or after a set of targets was observed, but before the grating was moved to another configuration.

Observing sequences were defined as several (2-5) repeats of ABBA sequences, with a 4" offset between the A and B positions. This was set as an offset perpendicular to the long axis of the IFU field-of-view, large enough to move from a centred object completelly off to sky (although in some of the cases where the seeing was really poor it was still possible to detect the wings of the PSF in the B position). On-target efficiency with this setup is reduced by 50%, but it avoids the problem of overlapping PSF wings due to the small size of the IFU if trying to dither on source.

Exposure times were calculated using the GNIRS ITC for two cases: (a) the maximum exposure time that would not saturate a single exposure (1 coadd) under IQ=70%, CC=50% conditions; and (b) the integration time per exposure required to obtain the desired signal to noise under IQ=Any, CC=90% conditions. A large number of coadds (rather than a longer integration time per exposure) was used to go from (a) to (b), thus avoiding the risk of saturation if observations were carried out under variable conditions (for example, clear patches between clouds). The same procedure was used to define the telluric standard observations.

The GNIRS data frames as retrieved from the Gemini Science Archive are in the standard Gemini MEF (Multi-Extension Format), where the primary header unit (PHU, extension [0]) includes all header information from telescope, environmental monitoring system and instrument; and the data extension [1] contains the pixel values.

Data reduction was performed using the tasks in the gemini.gnirs IRAF package, release Version 1.9, of July 28, 2006, and comprised the following steps:

Calibrations - Flats and arcs

  1. nsprepare: reformats the files to add the IFU Mask Definition File and applies the linearity correction to the data. The resulting file contains the PHU, one binary table extension with the MDF and one data extension [SCI,1] with the actual pixel values.
  2. nsreduce: cuts each of the 21 IFU slices according to the MDF inserted above to a separate SCI extension. No dark correction is applied to either flats or arcs.
  3. nsflat: combine the ten frames by extension, using ccdclipping for rejection and normalizing by the median of the illuminated area in each slice (as defined by the MDF). In average, the processing resulted in a S/N ~200-300 for each extension, with exception of slices 1 and 21 (which were partly vignetted) and slice 13 (which was damaged).
  4. used gemcombine to average the three processed arc frames to improve visibility of faint lines.
  5. nswavelength: obtain wavelength solution from combined arc. Using the Ar lamp, there are four lines in the "red" setting, and six in the "blue" setting. A low order polynomial (legendre order=3) was used, with residuals of the order of 0.15Å or smaller.

Science data and telluric stars

  1. nsprepare: reformats the files to add the IFU Mask Definition File and applies the linearity correction to the data. The resulting file contains the PHU, one binary table extension with the MDF and one data extension [SCI,1]
  2. nsreduce: cuts each of the 21 IFU slices according to the MDF inserted above to a separate SCI extension. Subtract adjacent pairs of object-sky frames and divide by the flatfield.
  3. nsstack: since we had only one position with actual data (the B position was blank sky), and the target objects were bright point sources observed under poor seeing conditions, we simply stacked all A  positions without any effort to improve alignment of the individual object frames by shifting according to the offsets registered in the headers. In most cases all frames were within 0.3arcsec tolerance (according to the offsets registered in the headers), but there were a few observations where drifts of up to 0.8arcsec were seen (usually due to clouds or very poor seeing affecting guiding performance).
  4. nstransform: applied the wavelength transformation to the stacked frame.
  5. nsextract: interactively extracted the spectrum from each slice, in order to exclude those with very low signal (the targets was not always well centred), the two edge slices and the damaged slice when the spectrum happened to fall within the damaged region. The output from this task is still a MEF file, with each SCI extension containing a 1D spectrum.
  6. used a simple cl script wrapped around specred.scombine to combine the valid spectra obtained in step 5. With this step, a single 1D standard FITS spectrum is created, but most of the information contained in the PHU of the MEF files is lost (scombine propagates the header of the first extension included in the combining list).
  7. finally, for the science data, applied the telluric correction using the standard specred.telluric task.
  8. combined the telluric-corrected spectra for those targets observed more than once.
  9. added back the header information lost in step 6, corresponding to the content of the PHU of the corresponding MEF frame obtained from step 5. (new on Version 1.5)

One additional step was applied to the data presented here, which was to remove the continnum shape by fitting a low order polynomial to the final telluric corrected spectrum.


The NIFS Sample:


The data were obtained with NIFS on the Gemini North telescope with the K_G5605 grating and HK_G0603 filter, resulting in a FWHM for the arc lamp lines of ~3.2Å. Each observation consisted of five individual exposures, with the star centred on the array then offset to each corner. The table below lists the observations, where for each object we present the date(s), associated programme ID, total exposure time on-source and the hot star used to correct for the telluric lines.

Star UT date ProgID
Exp time
NIFS Sample 1 (V1.5)
HD210885   20071015 06B-Q-107 150  HIP109911
HD107467  20060118 06A-SV-123 225 HIP56147
HD105028 20070430 07A-Q-25 30 HIP55564
HD10598 20061230 06A-SV-123 225 HIP8535
BD+44337 20061230 06A-SV-123 150 HIP7291
HD109655 20060201 06A-SV-123 50 HIP6141
HD3989 20071015 06A-SV-123 150 HIP4129
HD30354 20070130 06A-SV-123 675 HIP22348
HD236791 20070102 06A-SV-123 75 HIP7291
HD27796 20061230 06A-SV-123 75 HIP20674
HD235774 20071015 06A-SV-123 300 HIP109911
NIFS Sample 2 (V2.0)
HD109053 20070624 07A-Q-45 48 HIP68120
HD139195 20060722 06A-C-11 40 HR5685
HD108164 20060213 06A-C-11 540 HIP59394
HD124440 20070504 07A-Q-45 50 HIP68120
HD162211 20060722 06A-C-11 160 HIP93194
HD166229 20060722 06A-C-11 40 HIP93194
HD339034 20070505 07A-Q-62 160 HIP96153
HD129975 20070508 07A-Q-62 480 HIP72220
HD121447 20060213 06A-C-11 360 HIP59394
HD613 20071005 07A-Q-62 480 HD221491
HD181596 20060722 06A-C-11 120 HIP95853
BD+44 337 20071006 07A-Q-62 900 HD14212
HD201065 20060722 06A-C-11 80 HIP111169
Ves145 20070624 07A-Q-62 48 HD186440
BD+03 2954 20070603 07A-Q-45 660 HIP50459
HD118290 20070503 07A-Q-62 40 HIP65198
BD+09 4750 20071004 07A-Q-62 40 HIP196544
BD+59 274 20071004 07A-Q-62 340 HIP5361
BD-01 3097 20070508 07A-Q-62 200 HIP74689
BD+39 4208 20070502 07A-Q-62 40 HIP99359

The data reduction was accomplished using tasks in the gemini.nifs IRAF package. The reduction procedure included trimming of the images, flat-fielding, sky subtraction, wavelength and s-distortion calibrations. We have also removed the telluric bands and flux calibrated the frames by interpolating a black body function to the spectrum of the telluric standard star. Finally, the continuum shape was removed from the spectruum of each star (using the IRAF task continuum), normalizing all fluxes to unity.

Similar to the GNIRS spectra, the extraction procedure results in loss of the primary header content. This was added back to the spectra presented here in order to propagate the relevant instrument/telescope information.