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A Library of High-Resolution Spectra of Stars across the Hertzsprung-Russell Diagram S. Bagnulo, E. Jehin, C. Ledoux, R. Cabanac, C. Melo, R. Gilmozzi (ESO) and the ESO Paranal Science Operations Team Full text in pdf Contact persons: S. Bagnulo, R. Cabanac, E. Jehin, C. Ledoux, C. Melo, E-mail to all

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INDEX

• Introduction
• Rationale and scope
• Instrument Setup
• Data reduction
• Available data: the POP stars
  1. IC 2391
  2. NGC 6475
  3. Field stars
  4. Brightest stars of southern sky
• Legend of the Table of the available data
• Spectrum preview interface
• References
• Related links

INTRODUCTION

Program stars fall into two groups, namely stars belonging to open clusters, and field stars.
Spectra have been obtained for the largest possible number of members of two selected open clusters, IC 2391 (Omi Vel Cluster) and NGC 6475 (Messier 7), without compromising the quality of the spectra (in terms of S/N ratio). Open cluster stars were generally observed during night time.
Field stars are observed during twilight, and are selected to cover the largest possible variety of spectral types in the Hertzsprung-Russell Diagram. In order to reach a typical S/N ratio of a few hundred in V within the short interval of time available during twilight, the limiting magnitude for field stars was set to be V=7.5.

Telescope time for the POP was granted under DDT Prog. ID 266.D-5655, and the official title of the project is "An Atlas of High-Resolution Spectra of Stars across the Hertzsprung-Russell Diagram".

RATIONALE AND SCOPE

Open cluster stars

A similar exercise was repeated toward the end of P67, in August 2001, and the selected target was NGC 6475 (Messier 7). Compared to P66, less telescope time was available and it was not possible to observe extensively the cooler regions of the main sequence.

In total 48 stars of IC 2391 and 31 stars of NGC 6475 have been observed so far.

Field stars

INSTRUMENT SETUP: Instrument setups and exposure times

Setting Name	Cross Disperser	Filter	Central Wavelength	Slit Width	Slit Length	CCD Gap
DIC 1	CD #1	CuSO4	346 nm	0.5"	10"	-
DIC 1	CD #3	SHP700	580 nm	0.5"	12"	5 nm
DIC 2	CD #2	CuSO4	437 nm	0.5"	10"	-
DIC 2	CD #4	OG590	860 nm	0.5"	12"	10 nm

Before December 2001 the filter CuSO4 was used instead of HER-5.

All together, the two setups permit one to observe the wavelength intervals given in the following table, (the output of the UVES Exposure Time Calculator [ETC] is given for a V = 6 mag A0V star for each setting. It provides the details of the wavelength intervals per order for each setting):

wavelength	dichroic
304 - 388 nm	DIC1 346 blue arm
373 - 499 nm	DIC2 437 blue arm
476 - 577 nm	DIC1 580 red arm L
584 - 684 nm	DIC1 580 red arm U
660 - 854 nm	DIC2 860 red arm L
866 - 1040 nm	DIC2 860 red arm U

Almost the full wavelength interval from 300 to 1000 nm is observed, with a resolution of about 80,000. However, it should be noted that unfortunately the calcium triplet at 860 nm is in the gap between the two CCDs in the red camera.

We aim to get spectra with the highest reasonably possible S/N ratio. For bright stars (V<8), we want to obtain a S/N = 200 at 350 nm, and S/N = 200 at 1000 nm (which implies a higher S/N in intermediate bands, generally > 400-500). The exposure time is calculated taking into account the star's spectral type and is adjusted if needed during the observation to reach the highest signal without saturating the detectors.

DATA REDUCTION

The main characteristics of the method used for the reduction of these data include:

All observations and calibrations taken during an interval of time when the instrument was stable (i.e., no earthquake events, no technical interventions) are identified and further split in periods not longer than one week. All raw data which belong to a given period of time are reduced using a common set of bias and flat field calibration files. Therefore, the number of biases and flat-field images used for the creation of master frames for a given period of time has been increased from a standard of 5 to at least 15. This is needed to avoid the degradation of the high signal-to-noise ratio of the raw data. A master flat is created for each science night using the format check, order definition and wavelength calibration corresponding to the observing night. When important calibrations are missing (e.g. Th-Arg. lamps) in the raw data deliveries we download them directly from the online archive.

A routine to automatically detect of saturated raw science (and flat-field) images has been implemented. Indeed, as we are trying to reach the highest signal-to-noise ratios during the observations, it is not uncommon to end up with the saturation of the first spectrum of a series.

A MIDAS routine corrects bad CCD columns for all calibration and science frames (especially significant in the case of the lower Red CCD). The bad columns are replaced by the average of the two adjacent columns.
A table of wavelength ranges affected by flattening artefacts are given on the UVES QC page (from spectra taken with the same settings as this project): flag.txt.

The UVES pipeline average extraction method is used because it provides the best results for high S/N data. A subtraction of the sky spectrum is applied to the extracted object spectrum.

We have adopted an improved method of order mergingi. This is based on the rejection of a different number of pixels in, respectively, the blue and red overlapping ends of adjacent orders then the one used in the UVES pipeline. After careful inspection, we have indeed noticed the need for the rejection of a large number of pixels on the blue side of the orders (that are quite noisy and have a repetitive bad feature), whereas on the red side of the orders a more limited rejection is required. The optimal sizes in Å of the wavelength intervals to be skipped have been determined for the blue and red ends of the object orders for each instrumental setting.

For each observed star we perform an optimized co-addition (with cosmics rejection) of all the individual 1-D spectra per setting, producing one high signal-to-noise ratio spectrum per star per setting. During this process, the wavelengths are rebinned to heliocentric values.

An atmospheric extinction correction is applied, and a relative flux calibration is done for each setting using QC master response curve. More on the response curves may be retrieved from The ESO UVES QC page.

The master response curves used for the UVES POP settings are fitted + rebinned to the step of UVES spectra + normalized to unity:

A global merging (optimised for the overlapping regions) of the six final spectra is then performed in order to produce one final 1-D spectrum per star covering the whole optical range (300 to 1000 nm).

AVAILABLE DATA: THE POP STARS

Table 1: Stars from the open cluster IC2391 (Omi Vel Cluster)

Table 2: Stars from the open cluster NGC6475 (Messier 7)

Table 3: Field stars across the H-R Diagram

Table 4: The Brightest stars of the southern sky

SPECTRUM PREVIEW INTERFACE

UVES POP RELATED PAPERS

• Messenger no. 114, Dec 2003 in PDF The official release of UVESPOP in the ESO messenger in PDF.
  
• ADS citations to the messenger paper (incomplete )
• Castelli, F.; Hubrig, S., 2004, The Ca II infrared triplet as indicator of anomalous Ca isotopic mixture in HgMn stars, A&A, vol 421, L1-L4.
• Wahlgren, G. M.; Hubrig, S., 2004, Emission lines in the optical spectrum of Cen A. A&A, vol 418, 1073,1081.
• Cowley, C.R., Hubrig, S., 2005, A&A 432, L21-24
• Yushchenko, A, Gopka, V., Kim, C., Musaev, F., Kang, Y.W., Kovtyukh, V., Soubiran, C., MNRAS, 359, 865

RELATED LINKS

Send comments to <cabanac@cfht.hawaii.edu> Last modified: Tues. 16 October 2005