Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.mso.anu.edu.au/~bessell/FTP/Oldspectrophotometry/aa_paper_on_spectrophotometry.pdf Äàòà èçìåíåíèÿ: Tue Nov 30 02:01:57 2004 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 12:54:42 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: m 101

PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC, 111 : 1426 õ 1433, 1999 November ( 1999. The Astronomical Society of the Paciïc. All rights reserved. Printed in U.S.A.

Spectrophotometry : Revised Standards and Techniques
MICHAEL S. BESSELL
Research School of Astronomy and Astrophysics, Institute of Advanced Studies, The Australian National University, Private Bag, Weston Creek P.O., ACT 2611, Australia ; bessell=mso.anu.edu.au Received 1999 July 28 ; accepted 1999 July 29

ABSTRACT. The telluric features redward of 6700 A have been removed from the accurate spectro photometric standards of Hamuy et al. to permit more reliable relative and absolute spectrophotometry to be obtained from CCD spectra. Smooth ÿuxes from 3300 to 10500 A are best determined by dividing the raw spectra of all objects taken in a night by the raw spectrum of a "" smooth îî spectrum star before deriving the instrumental response function using the revised standard star ÿuxes. In this way the telluric features and any large instrumental variation with wavelength are removed from the raw data, leaving smooth spectra that need only small corrections to place them on an absolute ÿux scale. These small corrections with wavelength are well described by a low-order polynomial and result in very smooth ÿux-calibrated spectra.

1. INTRODUCTION High-quality CCD spectra are now routinely obtained for stars and galaxies at most observatories. However, ÿux calibration of these spectra has not been universal, nor has it been very accurate when done, because of the paucity of suitable accurate spectrophotometric standards. Another problem with CCD spectroscopy has been the reluctance of many observers to remove the eects of atmospheric (telluric) absorption from their red spectra, as can often be seen in published data. This makes the identiïcation of weak stellar features difficult and the computation of synthetic colors and magnitudes imprecise. The recent publication of precise standards by Hamuy et al. (1994) is a major contribution to precise spectrophotometry ; however, they have also neglected to remove the telluric absorption in their standard ÿuxes, which unnecessarily degrades the precision of ÿux calibration. The problem is that the atmospheric absorption is not constant. Some of the bands, particularly the O bands, are almost saturated, but the H O 2 2 bands are only partially saturated and vary with humidity and with some power of the air mass. It is much better to remove the atmospheric absorption from the raw spectra and use the telluric-free absolute ÿuxes for calibration.

2. CORRECTED FLUXES FOR THE HAMUY ET AL. STARS The list of Hamuy et al. (1994) spectrophotometric standards comprised a set of 10 bright A-type stars at 16 A spacing and a set of 19 fainter white dwarf and subdwarf stars at 50 A spacing. I have removed the telluric absorption separately from the two sets. 1426

For some of these stars I had low-dispersion spectra in which the telluric bands had been removed using the techniques described in ° 6. These showed that Feige 110 and some other stars have completely smooth spectra beyond Ha. Consequently, a third-order polynomial was ïtted to the highest points in the Hamuy et al. spectrum of Feige 110 redward of 6700 A, and this continuum was taken to be the "" true îî spectrum. These magnitudes were then subtracted from the original spectrum and provided the ïrst estimate of a set of magnitude corrections. This correction was applied to several other of the smooth stars and the continuum similarly ïtted. In the G subdwarfs the absorption lines due to the Ca II triplet were reinstated below the smooth continuum. Finally, an average correction derived from the subdwarfs and the continuous white dwarfs was applied to those hotter stars showing Paschen lines and a Paschen jump, and a polynomial was ïtted between 6700 and 8400 A. Above the Paschen jump the continuum between the Paschen lines was made smooth. In Figure 1 are plotted all the dierences between the originally published spectra and the "" telluric-free îî spectra. The variations in the H O absorption bands from spectrum to spec2 trum can be seen. One star, LTT 3218, had signiïcantly larger residuals than the average, indicating that it may have been observed at larger air mass. The 19 telluric-free spectra are available together with the set of average magnitude corrections by anonymous ftp.1 For a ïrst-order correction to the bright A-star spectra, the average telluric spectrum at 50 A spacing was interpolated at 16 A spacing and subtracted from the spectra of the hotter stars HR 3454
õõõõõõõõõõõõõõõ 1 Available from mso.anu.edu.au at /pub/bessell/ or from http :// www.mso.anu.edu.au/Dbessell/FTP/.


SPECTROPHOTOMETRY

1427

FIG. 1.õPlot of telluric absorption (revised minus original magnitudes) for the faint standards of Hamuy et al. (1994). Data are in 50 A bins.

FIG. 2.õSame as Fig. 1 but for the bright A-star standards of Hamuy et al. (1994). Data are in 14 A bins.

and HR 9087. The resultant spectra had a polynomial ïtted to the continua between 6800 A and the Paschen jump at 8400 A, and a smooth gradation was ensured for the contin uum between the Paschen lines by hand correction. These corrected spectra were then subtracted from the two original spectra directly, producing a 16 A spaced set of correc tions. This average correction was then subtracted from all the other stars. For some of the stars the correction was nearly perfect, but in several the correction was insufficient, requiring additional correction between 9300 and 9500 A. Polynomial ïts were used for all stars between 6800 and 8400 A. In Figure 2 are plotted the individual dierences for all the bright stars. There is some scatter in the H O fea2 tures, but it is less than for the faint group of standards. The telluric-free spectra of the 10 HR stars are also available together with the set of average magnitude corrections by anonymous ftp. There are also some very weak atmospheric H O bands 2 between about 5900 A and Ha, but these have been

removed from only four of the DA white dwarfs where it is certain that there are no stellar features.

3. SPECTROPHOTOMETRIC STANDARDS In Table 1 are listed the coordinates and UBV RI colors in the Cape-SAAO UBV RI system for the bright standards. In Table 2 the colors and coordinates for the 19 fainter Hamuy et al. standards are given along with several other stars useful for spectrophotometry. The Hamuy et al. stars were selected from the tertiary standard lists of Baldwin & Stone (1984), Stone (1977), and Massey et al. (1988) and from the secondary standard list of bright stars by Taylor (1984) ; see Hamuy et al. (1992) for details. Other lists of spectrophotometric stars have been given by Oke (1974 ; white dwarfs), Oke & Gunn (1983 ; subdwarfs), Bartkevicius & Sviderskiene (1981 ; metal-poor dwarfs and giants), Massey et al. (1988 ; white dwarfs and sdO stars), and

TABLE 1 BRIGHT A-STAR SPECTROPHOTOMETRIC STANDARDS Spectral Type B9 III A1 Vn B3 V B9.5 Vn A1I V B9.5 V A0 III A1 V B8 V B9 III B[V [0.056 0.02 [0.200 [0.08 [0.01 [0.023 0.10 [0.001 [0.09 [0.136 U[B [0.107 [0.03 [0.743 [0.18 [0.01 [0.080 [0.02 0.029 [0.27 [0.501 V [R [0.023 0.014 [0.083 [0.023 0.003 ... ... [0.005 ... [0.052 V [I [0.063 0.039 [0.200 [0.063 0.010 ... ... [0.010 ... [0.122 R.A. (J2000) 02 04 08 11 13 14 19 20 22 00 28 50 43 36 09 45 54 47 41 01 09.9 36.7 13.4 40.8 56.9 30.1 44.7 40.5 27.6 49.3 Decl. (J2000) 08 08 03 [09 [05 00 00 [09 10 [03 27 54 23 48 32 43 16 29 49 01 36 01 55 08 20 02 25 45 53 39

HR 718 ....... 1544 ...... 3454 ...... 4468 ...... 4963 ...... 5501 ...... 7596 ...... 7950 ...... 8634 ...... 9087 ......

Name m2 Cet n Ori g Hya h Crt h Vir 108 Vir 58 Aql v Aqr f Peg 29 Psc

HD 15318 30739 74280 100889 114330 129956 188350 198001 214923 224926

V 4.279 4.355 4.295 4.70 4.375 5.681 5.62 3.778 3.40 5.120

NOTE.õUnits of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds.

1999 PASP, 111 : 1426 õ1433


1428 BESSELL

TABLE 2 FAINT SPECTROPHOTOMETRIC STANDARDS

Star 0046]051 0310[688 L995-86 L894-1 0738[172 0839[327 1142[645 1236[495 LHS 2594 L916-15 LHS 405 1620[391 1917[077 2007[303 2316[173 2317[054 L877-23 L822-50 [44¡12736 LDS 678A LHS 235 LHS 253 L465-10 LHS 43 [34¡239 LHS 7 [28¡595 sdG DZ7 sdG DA3 sdG sdG B1 V DZQ6 DA sdG DQ6 B5p DA6 A0 a sdG DA2 sdG DBQ5 DA4 sdG DB3 sdO 11.225 12.382 11.511 11.380 13.153 12.210 10.42 13.03 11.846 12.164 11.512 11.11 13.782 10.441 11.812 7.212 11.012 10.227 12.28 12.217 12.062 14.104 11.846 0.488 0.550 0.570 0.036 0.485 0.413 ... 0.245 0.229 0.506 0.190 ... 0.177 0.339 0.488 0.499 [0.132 0.616 0.04 0.060 0.629 0.029 [0.304 0.295 0.255 0.356 [0.087 0.304 0.270 ... 0.154 0.088 0.322 0.161 ... [0.013 0.166 0.319 0.331 [0.125 0.354 0.08 [0.096 0.384 0.023 [0.133 0.587 0.499 0.715 [0.180 0.635 0.559 ... 0.310 0.203 0.662 0.295 ... 0.007 0.329 0.659 0.688 [0.267 0.707 ... [0.178 0.780 0.071 [0.325 00 00 01 03 03 05 06 07 08 10 11 12 12 14 15 15 16 18 19 20 22 23 23 41 49 54 10 48 56 45 40 41 32 45 06 38 11 38 43 23 36 20 10 52 19 19 47 10 48 30 24 25 14 21 31 15 43 40 46 46 59 03 34 26 35 56 41 35 58 [0.058 0.006 [0.193 [0.671 [0.273 [0.201 ... [0.58 [0.531 [0.157 [0.674 ... [0.646 0.118 [0.203 [0.203 [0.949 [0.016 0.04 [0.632 [0.118 [0.831 [1.153 [33 ]05 [27 [68 [39 [27 ]02 [17 [32 [35 [64 ]11 [49 [33 [28 [10 [39 [44 [07 [30 [20 [17 [05 39.2 23.4 28.7 36.1 08.6 51.5 08.3 24.9 56.4 37.5 50.4 40.3 49.1 03.2 35.5 56.0 13.9 18.7 40.1 13.1 35.4 05.4 09.9

WD

Other

Spectral Type V B[V U[B V [R V [I

R.A. (J2000)

Decl. (J2000)

k yr~1 a (arcsec) [0.030 0.0812 0.0225 0.0090 0.0176 0.0194 ... 0.0785 [0.0836 [0.0229 0.4154 ... [0.0574 0.0005 [0.0173 [0.0776 0.0068 [0.0145 [0.0042 [0.0285 0.0059 0.0174 ...

k yr~1 d (arcsec) [0.245 [2.722 [0.201 [0.075 [0.191 [0.173 ... [0.548 1.347 0.015 [0.325 ... [0.128 [0.019 [0.179 [0.317 [0.008 [0.156 [0.190 [0.240 [0.309 0.022 ...

Reference H O H H H H H O, H H H H H H H H O, B H H G, M H H H, O H, O

LTT 377 ............ VMa 2 .............. LTT 1020 ........... EG 21 ............... LTT 1788 ........... LTT 2415 ........... H600 ................ L745-46A ........... LTT 3218 ........... LTT 3864 ........... LTT 4364 ........... Feige 56 ............. LTT 4816 ........... CD [32¡9927 ...... LTT 6248 ........... HD 140283 ......... EG 274 ............. LTT 7379 ........... EG 131 ............. LTT 7987 ........... LTT 9239 ........... LTT 9491 ........... Feige 110 ...........

NOTE.õUnits of right ascension are hours, minutes, and seconds, and units of declination are degrees and arcminutes. Spectrophotometric data from H : Hamuy et al. (1994) ; O : Oke (1974), Oke & Gunn (1983), Oke (1990) ; B : Bartkevicius & Sviderskiene (1981) ; G : Greenstein (1984) ; M : this paper

1999 PASP, 111 : 1426 õ1433


SPECTROPHOTOMETRY
TABLE 3 STARS WITH NEAR-BLACKBODY FLUXES Spectral Type DZ7 DZQ6 DA1 DBQ5 DQ6 sdO

1429

Star VMa 2 ......... L745-46A ...... HZ43 .......... EG 131 ........ L1363-3........ F110 ...........

WD 0046]051 0738[688 1314]293 1917[077 2140]207 2317[054

T BB 6650 8600 35000 11800 9500 55000

mag AB(5550 A) ' 12.36 13.03 12.99 12.27 13.22 11.85

Absorption Features Shortward of 4100 A HK Ca II 3850õ 4050 A Weak H lines None Weak C 4600õ5300 A 2 Weak H and He lines

Greenstein (1984 ; white dwarfs) ; the additional stars in Table 2 are taken from these papers. The magnitudes given by Oke & Gunn (1983) for the wavelength points longward of 9000 A in HD 140283 are incorrect. They should be 9300 A : 6.80 ; 9700 A : 6.79 ; 9900 A : 6.78 ; 10200 A : 6.78 (Bartkevicius & Sviderskiene 1981). UBV RI photometry for

FIG. 3.õOriginal and revised magnitudes for LTT 9239

Table 2 has been taken from Kilkenny & Menzies (1989) with some additional stars measured by the author ; Landolt (1992) has also measured many of the stars. UBV RI photometry for Table 1 is from Cousins (1971, 1984 ; UBV ) and Cousins (1980 ; VRI). The WD identiïcation in Table 2 is from McCook & Sion (1999). The LHS identiïcation, coordinate, and proper motions are from Luyten (1979a) ; the L and LTT identiïcations, coordinates, and proper motions are from Luyten (1979c). Finding charts are discussed in a later section. The published spectrophotometric data for the white dwarfs L745-46A and VMa 2 have lower precision than for the Hamuy et al. (1994) stars. Until better data are available, it may be better to use blackbody ïts to their continua. The ÿuxes for EG 131 have been derived by the author. Table 3 lists blackbody temperatures, AB magnitudes at 5500 A, and comments on any weak features for stars with near-blackbody ÿuxes. The ïtted blackbody ÿuxes are also available by anonymous ftp. Spectrophotometric standard ÿuxes have also been presented by Oke (1990) for 25 white dwarfs and subdwarfs for the Hubble Space T elescope. Filippenko & Greenstein (1984) also published ÿuxes for four of these stars.

FIG. 4.õOriginal and revised magnitudes for EG 21

FIG. 5.õMagnitudes for the new standard EG 131

1999 PASP, 111 : 1426 õ1433


1430

BESSELL L745-46A and VMa 2 are especially useful in the red as they have no bands or lines redward of 4100 A and are smooth blackbodies. Other suitable subdwarfs are given in Table 2. At very high resolution (0.1 A), many sharp lines can be seen in the spectra of these metal-poor stars, so for removal of telluric lines in high-resolution spectra it is best to use fastrotating B stars. 5. BLUE STARS FOR INSTRUMENTAL RESPONSE NORMALIZATION AND UBVRI SYNTHETIC PHOTOMETRY Although there are no telluric lines between 3400 and 5500 A, it is still useful to have smooth template stars that can be used between these wavelengths to remove the instrumental response, which is often changing rapidly with wavelength below 4000 A. The Huggins ozone bands below 3400 A can also best be removed using stars that are bright and smooth in the UV ; hot DA white dwarfs are ideal for ozone removal (Schachter 1991) but are not as useful for

Figures 3 and 4 shows two sample spectra of standard stars with and without telluric correction. Figure 5 shows the spectrum of the new standard EG 131. 4. SMOOTH SPECTRUM STARS FOR TELLURIC LINE REMOVAL As will be described below, extremely metal-poor G dwarfs and red giants make good smooth templates for low-resolution spectrophotometry for wavelengths redward of B5500 A. The best stars are those with metallicities below [2 dex and with temperatures between 5000 and 6000 K. They have essentially smooth spectra at low resolution (B10 A) apart from weak Ha and weak Na D or Ca II lines. A list of suitable stars for telluric line elimination is given in Table 4. These have been selected from the photometry of Norris, Bessell, & Pickles (1985) and the papers of Oke & Gunn (1983) and Bartkevicius & Sviderskiene (1981). Many of the extreme metal deïcients stars were identiïed by Bond (1980). The bright He-rich white dwarfs

TABLE 4 SUITABLE LOW-RESOLUTION SMOOTH SPECTRUM STARS Spectral Type Giant Giant Giant DG Dwarf Giant Giant DF Giant Giant Dwarf Giant Giant Dwarf Giant Giant Giant Giant Dwarf Dwarf Giant DC Giant Giant Giant Giant Giant Dwarf Dwarf Giant R.A. (J2000) 00 00 00 00 03 04 05 07 07 09 09 09 10 10 12 12 14 14 14 15 18 19 19 19 19 20 20 22 23 23 30 31 45 49 08 00 10 40 53 47 48 53 14 51 04 07 02 27 49 43 04 20 37 55 58 42 57 11 17 29 45.4 16.9 27.1 10 25.6 52.5 49.4 21 33.1 19.2 56.1 39.2 29.0 28.1 43.1 53.1 31.8 00.4 02.4 03.1 40.0 35.0 11.9 09.7 49.7 48.8 27.4 31.4 05.0 28.8 Decl. (J2000) ]57 [16 [09 ]05 ]26 [75 [37 [17 ]30 [41 ]13 [22 ]53 ]20 [29 [38 ]09 [22 ]25 [10 ]03 [07 [39 ]10 [18 [20 [36 ]18 [13 ]30 03 47 32 23 19 36 49 24 36 27 44 50 33 16 11 40 41 14 42 56 46 40 44 44 12 00 32 05 51 25 53.7 40.8 40.0 24.0 54.9 11.4 06.9 54.0 17.9 04.5 39.3 08.4 59.2 39.0 05.6 25.3 09.6 39.1 08.8 00.7 44.8 06.0 37.5 27.3 11.3 39.4 53.3 33.6 03.9 57.7 k yr~1 a (arcsec) 0.0051 [0.0003 0.0037 0.0812 [0.0149 0.0374 ... 0.0785 0.0561 [0.0007 0.0255 [0.0010 0.0005 [0.0188 [0.0025 [0.0009 [0.0131 [0.0009 [0.0002 [0.0759 [0.0016 [0.0042 0.0002 [0.0025 0.0005 [0.0011 0.0024 0.0359 [0.0340 [0.0015 k yr~1 d (arcsec) [0.064 [0.051 0.016 [2.722 [0.795 0.077 ... [0.548 [1.834 [0.007 [0.775 [0.018 [0.031 [0.453 [0.013 [0.032 [0.075 [0.049 [0.351 [0.302 [0.075 [0.190 [0.061 0.289 [0.091 [0.007 [0.044 0.049 [1.192 [0.055

Star HD 2665 ............. HD 2796 ............. HD 4306 ............. VMa 2 ................ HD 19445 ............ HD 26169 ............ HD 33771 ............ L745-46a ............. HD 64090 ............ HD 84903 ............ HD 84937 ............ HD 85773 ............ HD 88609 ............ HD 94028 ............ HD 104893 .......... CD [37¡7677 ....... HD 122563 .......... HD 126587 .......... BD ]26¡2606 ....... HD 140283 .......... HD 165195 .......... EG 131 ............... HD 184711 .......... HD 188510 .......... BD [18¡5550 ....... CD [20¡6008 ....... CD [37¡14010 ...... BD ]17¡4708 ....... HD 219617 .......... HD 221170 ..........

V 7.7 8.5 9.0 12.4 8.0 8.9 9.5 13.0 8.3 8.0 8.1 9.4 9.2 8.2 9.2 9.9 6.2 9.1 10.1 7.3 7.4 12.3 8.0 8.8 9.3 9.8 9.7 9.4 8.2 7.7

Reference B

O O, B

O, H B O, B

B

B O O, B B

B

O, B B B

NOTE.õUnits of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. Spectrophotometric data from O : Oke (1974), Oke & Gunn (1983) ; B : Bartkevicius & Sviderskiene (1981) ; G : Greenstein (1984).

1999 PASP, 111 : 1426 õ1433


SPECTROPHOTOMETRY response normalization redward of the Balmer jump. In Table 5 are listed some hot stars that are excellent for this purpose. The best star is EG 131, a bright DC He-rich white dwarf which has unfortunately been overlooked by all those setting up standards because it was erroneously classiïed as DAwk. This 12th magnitude star has no lines in its spectrum between 3000 and 11000 A and has a blackbody spec trum. It can also obviously be used in the red. Subdwarf O stars such as Feige 110 that have virtually no Balmer discontinuities are the next best stars with which to remove the often rapid changes with wavelength in the UV-blue sensitivity function. Some of the sdO stars are from a list of spectrophotometric standards by Massey et al. (1988). The extremely blue stars are also very important for standardizing the far-blue end of the UBV RI system when making synthetic photometry. The UBV RI colors are from Menzies, Marang, & Westerhuys (1990). The bright A stars in Table 1 should be used for UBV RI standardization in the important region (U[B, B[V ) \ (0, 0) where stars have large Balmer jumps. 6. OBSERVING AND REDUCTION STRATEGIES The observing technique that is recommended is to observe a series of "" smooth îî spectrum stars during the

1431

night at air masses encompassing those for the program stars. Some of these smooth stars may also be spectrophotometric standards. One of these spectra or the average of several spectra is then used as the template for division through all objects observed in the night at the same grating setting. Such a division serves two purposes. First, it removes most of the telluric absorption, and second, it removes any large variations along the spectrum due to detector sensitivity, grating efficiency, and spectrograph vignetting. Before division, the template spectrum should have any stellar lines removed. In the red, the cool metal-poor dwarfs and giants may show Ha, Na D (5889, 5896 A) and the Ca II (8498, 8542, 8662 A) triplet. The spectrum can also be smoothed in regions with no telluric lines. After division by the template spectrum, the spectra of other smooth spectrum stars taken at larger air mass or at a dierent time will likely show residual H O absorption at a few places. These 2 spectra should then be normalized and ÿattened so that their continua have a level of about 1.0, and in the regions where the telluric absorption was successfully removed the continua should be replaced by 1.0 exactly. The resultant normalized spectra or some power of the normalized spectra can then be divided into any other spectra that show evidence of insufficient H O correction without 2

TABLE 5 BLUE STARS FOR UV RESPONSE NORMALIZATION Spectral Type sdO sdO sdOB sdO sdO sdOB sdO sdO sdB sdOp sdO sdOp sdO sdO sdO ... sdO sdO sdO sdB sdO DC sdO ... ... B[V [0.316 [0.072 [0.303 ... [0.354 [0.268 [0.291 [0.306 [0.299 [0.299 ... [0.324 [0.221 ... ... [0.255 ... ... [0.260 [0.289 ... 0.04 ... ... [0.324 U[B [1.246 [1.043 [1.131 ... [1.229 [1.108 [1.173 [1.207 [1.140 [1.207 ... [1.219 [1.098 ... ... [1.160 ... ... [1.184 [1.108 ... 0.04 ... ... [1.166 V [R [0.084 0.048 [0.138 ... [0.182 [0.127 [0.113 [0.131 ... [0.131 ... [0.141 [0.072 ... ... 0.006 ... ... [0.115 [0.125 ... 0.08 ... ... ... V [I [0.178 0.131 [0.345 ... [0.343 [0.266 [0.260 [0.293 ... [0.316 ... [0.319 [0.188 ... ... 0.039 ... ... [0.276 [0.264 ... ... ... ... ... R.A. (2000) 00 00 01 02 03 05 06 07 08 08 08 09 09 09 10 11 12 13 14 16 17 19 21 22 21 47 52 43 19 33 15 48 36 02 35 26 07 25 38 39 50 41 23 32 34 09 20 51 59 12 03 15 51 19 14 00 05 30 15 20 50 07 36 20 37 51 49 35 21 23 16 35 11 38 19 Decl. (J2000) [11 [10 [24 ]03 [25 [08 [44 [32 [03 [01 ]54 [03 [28 ]55 ]43 [23 ]17 ]36 [22 [04 ]60 [07 ]28 [20 ]10 52 39 02 26 51 48 18 12 58 55 28 06 38 05 06 21 30 08 39 00 10 40 51 50 47 37 57 57 54 47 39 59 57 16 48 05 07 41 53 11 52 57 01 26 52 10 06 53 13 20

Name BD [12¡134 ........... BD [11¡162 ........... UV 0141[24 ........... PG 0216]032 .......... CD [26¡1339 .......... UV 0512[08 ........... HD 49798 ............... CD [31¡4800 .......... BD [03¡2179 .......... UV 0832[01 ........... PG 0823]546 .......... UV 0904[02 ........... CD [28¡7246SW ...... PG 0934]554 .......... Feige 34 ................. BD [22¡3230 .......... Feige 67 ................. HZ 44 ................... HD 127493 ............. HD 149382 ............. PG 1708]602 .......... EG 131 .................. BD ]28¡4211 .......... NGC 7293 .............. UV 2309]10 ...........

V 11.772 11.169 11.774 14.6 11.305 11.317 8.297 10.550 10.347 11.466 14.3 11.991 11.223 12.2 11.2 11.749 11.8 11.7 10.039 8.962 13.7 12.28 10.5 13.51 13.101

Reference

M

M

M M, O M, O M, O

M M O

NOTE.õUnits of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. Spectrophotometric data from M : Massey et al. (1988) ; O : Oke (1990).

1999 PASP, 111 : 1426 õ1433


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BESSELL although the H O band are susceptible to variation with 2 time. In addition, Schachter, Filippenko, & Kahn (1989, 1990) have emphasized the importance of not degrading the signal-to-noise ratio of the program stars observations by division by a template star. Rather than use the stellar spectrum directly, they approximate the featureless continuum with a cubic spline, thus interpolating over intrinsic and telluric absorption lines. They then set the spline continuum to unity in all regions away from the telluric features to produce a template. Reducing data in the way described above enables one in the blue to conïdently identify weak broad features such as emission in QSOs, interstellar absorption bands, cyclotron absorption in cataclysmic variables, Zeeman broadened hydrogen lines in magnetic white dwarfs, and carbon bands in cool He-rich white dwarfs. In the red, one can easily identify weak bands of CN, TiO, and ZrO bands in cool giant stars and the important bands of FeH in cool dwarfs. Most importantly, it enables one to rule out the presence of such features in spectra and thus avoid erroneous classiïcations.

changing the overall ÿux scale. See below for Wade & Horneîs (1988) appropriate sec z scaling. The extinction table to be used with the telluric-corrected spectra should correct only for the continuous absorption and scattering and not for the H O bands. The extinction 2 table of Hayes & Latham (1975) is suitable for such interpolation. Shortward of about 5000 A it is best not to use the metal poor cool stars as templates as they have many blended features. The main purpose of division in this spectral region is to remove the instrumental response, which often changes very rapidly between 3300 and 4300 A. The ÿat ïeld lamp useful in the red for this purpose is of little use here because most lamps provide insufficient UV light. As noted above, the DC white dwarf EG 131 is the best choice ; L745-46a and VMa 2 can be used redward of about 4100 A. Subdwarf O stars can be used quite eectively. Obvious hydrogen and helium lines are ïrst removed ; then the resultant spectrum is heavily smoothed before division to increase the eective signal-to-noise ratio. It is important to use spectra of stars without Balmer jumps so that the eect of the division is to produce a smooth transition through the region of the conÿuence of the Balmer lines. The results of red and blue divisions should be telluric line-free spectra showing only slowly varying changes of continuum intensity with wavelength. A comparison between these spectra and the standard ÿuxes will yield small magnitude dierences that vary only slowly with wavelength. Such dierences can be readily ïtted with a low-order polynomial which permits conïdent interpolation across fairly large wavelength intervals. This is necessary when the values derived from strong-lined standards at the conÿuence of the Balmer and Paschen lines are poorly determined or when data points are absent, such as in the many ÿux calibrations tabulated at the minimal "" Oke standard îî wavelengths. Low-order polynomials produce much better results than spline ïts. Others have paid careful attention to the removal of telluric lines and instrumental response function. The authorîs ïrst experience was in using the image dissector scanner (Robinson & Wampler 1972) on the Anglo-Australian Telescope in the early 1970s under the tutelage of J. Wampler. A blackbody spectra or a bright smooth starîs spectrum was stored on-line and divided through oneîs observations with a remarkable enhancement of the visibility of very weak features that had not been seen previously on photographic or image tube spectra of the objects. It was then natural to seek to emulate these excellent results when CCDs later came into regular use. Wade & Horne (1988) also discuss the importance of telluric absorption removal. They have found that the mean telluric magnitude dierences obtained near the zenith (such as in Figs. 1 and 2) can be scaled with increasing air mass as (sec z)0.6 and that this corrects both the O and H O bands, 2 2

7. IDENTIFICATION CHARTS Charts for all the Hamuy et al. stars can be found in Stone & Baldwin (1983). Luyten (1979b) has excellent charts for almost all the LHS stars, and Luyten (1949) provided charts for many of the brightest white dwarfs, but note that the high proper motion stars have moved signiïcantly from their positions in these charts and in the LHS charts which were made from plates taken in the 1950s. Probably the best way to make (unmarked) charts is to use the Canadian Astronomy Data Centre Web site2 and provide a ïle containing the name, right ascension, declination, epoch, and chart size for each star, one per line. A gif ïle for each star is generated with the scale attached. These gif ïles of the ïelds for each of the faint spectrophotometric standards (Table 2) and the blue stars (Table 5) are also available (faint.gif.tar and blue.gif.tar) from the authorîs anonymous ftp site.3 I would like to thank the referee Alex Filippenko for his helpful comments and Heath Jones for his advice on making ïnding charts.
õõõõõõõõõõõõõõõ 2 See the CADC Interface to the Digitized Sky Survey at http :// cadcwww.dao.nrc.ca/cadcbin/getdss. 3 See mso.anu.edu.au at /pub/bessell/ or http ://www.mso.anu.edu.au/ Dbessell/FTP/.

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