Two-dimensional spectral classification of F stars through photoelectric photometry with interference filters

1336

Two-dimensional spectral classification of" F stars

stars with the same ~1 and D co-ordinates whose points wou|d lie in the E-surface. Consequently, all the stars of population, 1I .]br which we actually have the three ~, D, and Ib lie outside the surface )2 (which explains why they cannot be classified in the MK-system): their points lie below the upper portion of E. 6. CONCLUSION The results which have been presented are as yet too incomplete for a general conclusion to be arrived at with any certainty, but they seem to indicate that, by using three suitably chosen parameters, it is possible to set up a classification applicable to a much greater number of stellar types than are covered by the older classifications. }~EFEREN( ES

BARBIEI¢, D. and CHALONGE, l) . . . . . BARBIER, D., CHALONGE, D. a n d MORGULEFF, NINA . . . . . . . . . BERGER, J., CANAVAGGIA,R., CHALONGE, D. and FRINGANT, ANNE-MARIE . . . . CnALONGE, D. and DIVAN, LUCIENNE HACK, MARGHERITA . . . . . . . JOHNSON, H. L. a n d MORGAN, W. V¢. . . MORGAN, W. I,V., KEENAN, 13. C. and KELLMAN. EDITH . . . . . . . . .

1939 1941

Ann. Astrophys., 2, 254. Ann. Astrophys., 4, 30.

194l

Ann. Astrophys., 4, 137.

1953 1952 1954 1953

Uontrib. Inst. Astrophy~'. Paris., Set'. A, No. 91. Ann. Astrophys., 15, 201. Ann. Astrophys., 1{1, 417. Ap. J., 117, 313.

1943

An Atlas of SteUar Spectra (University of Chicago

1948 1951

Ouspekhi Astron. Nauk, 4~ 69. Astron. J.. 56, 142.

Press). FARENAGO, P. P .

STROMGREN, B .

. .

. .

. .

. .

. .

. .

. .

. .

Two-dimensional Spectral Classification oi F Stars Through Photoelectric Photometry with Interference Filters B E N G T STROMGREN Yerkes and McDonald Observatories SUMMARY A m e t h o d of spectral classification of F stars t h r o u g h photo-electric p h o t o m e t r y with interference filters is described. Two classification indices are determined, one measuring the s t r e n g t h of the H/~ line, the other the Balmer discontinuity. Both indices are practically uninfluenced by interstellar reddening. Results of observations with the 82-inch reflector of the McDonald Observatory in F e b r u a r y a n d March, 1953, are compared with spectral classification b y W. W. MORGAN, with colour indices (B-V) on HAROLD JOHNSON'S system, and with absolute m a g n i t u d e s from trigonometric parallaxes and spectroscopic determinations. The photometric probable error (one observation) of a (B-V) value predicted from the two classification indices is ± 0~?006, corresponding to a probable error i 0-02 of a spectral class. The photometric probable error (one observation) of a predicted value of the visual absolute m a g n i t u d e My is i 0 ~?18. The comparison with the d a t a mentioned above indicates t h a t the influence of cosmical scatter on the predicted (B-V) a n d My values is small, or negligible. Applications in the s t u d y of galactic structure are v e r y briefly discussed.

1. INTRODUCTION METHODS f o r t w o - d i m e n s i o n a l photographed five years ago.

spectral

classification

with an objective prism were developed The classification was based

from

low-dispersion

spectra

b y B. LINDBLAD o v e r t w e n t y -

on intensities

of relatively

strong

line

BENGT STRSMGREN

1337

features in the spectra which could be evaluated even in the case of the short spectra used. It should be possible to measure the intensities of such strong line features with the help of interference filters transmitting spectral ranges of half-width less than 100 A, if the photometry were made photoelectrically. After a preliminary test carried out at the McDonald Observatory in 1948 (STRONGREN, 1950) had given promising results the author decided to investigate the possibilities of spectral classification through photoelectric photometry with interference filters in an empirical way. A photoelectric photometer with a filter-wheel containing twenty-six filters was constructed at the Copenhagen Observatory. Of the filters, twenty-five were interference filters made by Bausch & Lomb with transmission maxima in the range from 3800-5500 A, while one filter was a UV filter with transmission maximum at 3550 A. The half-widths of the interference filters ranged from 80-120 A; the maximum transmissions from 35-45 per cent. The filter photometer was tested at Copenhagen Observatory and then used extensively by Mr. K. GYLDENKARNEwith the 32-in. reflector of the Observatoire de Haute-Provence. Measures were made for 1 l0 standard stars of all spectral types for which spectral classes and luminosity classes had been determined by W. W. MORGAN. An analysis of the photometric material indicated that a number of intensity ratios gave indices useful in spectral classification (STROMOREN, 1951). For G and K stars a two-dimensional classification giving spectral class as well as luminosity class proved possible with the aid of the tbllowing intensity ratios : (1) 4320/4240, which measures the discontinuity at the G-band. (2) 3910/4030, which is a measure of the K4ine intensity. (3) 4240/4170, a cyanogen strength index. Either the first or the second index can be used together with the third index for two-dimensional classification. The first two indices have the largest variation with spectral class, while the third varies strongly with absolute magnitude. The three intensity ratios correspond closely to ratios used by LINDBLAD and his collaborators at the Stockholm Observatory in their photographic-spectrophotometric classification work. The work on G and K stars has been continued by Mr. K. GYLDENKARNE,Copenhagen Observatory, with a photoelectric photometer attached to a 10-in. reflector. The results will be published elsewhere. The methods for classification of B, A, and F stars have been further developed by the author at the McDonald Observatory (STR()MGREN, 1952). 2. THE CLASSIFICATION INDICES While the measures made at the Observatoire de Haute-Provence had confirmed the conclusion reached earlier (STR(:)~IGREN,1950) that a useful classification index could be derived with the help of interference filters at Hfl and at a comparison wavelength close by, they indicated that considerably more accurate results could be expected if a narrower Hfl filter were available for the purpose. A dielectric Hfl filter of half-width 35 A and maximum transmission about 75 per cent was obtained from Bausch & Lomb. When used together with a blue and a yellow filter, to cut out violet and red transmission, it gave a very satisfactory transmission curve. Two

1338

Two-dimensional spectral classification of F stars

metallic interference filters of the same type as those previously used served as comparison filters, their maximum transmissions being at 4700 A and 5000 A. The following index was used as a measure of the strength of Hfl, namely, 1 ~-- 2.5 (½[log 1(4700) ~- log 1(5000)] -- log I(4861)} • const . . . . .

(1)

where I(4700), I(5000), and I(486I) denote the intensities measured through the two comparison interference filters, and the Hfl interference filter, respectively. Since the average of the wavelengths of the comparison filters is very close to 486l, the influence of interstellar reddening upon the index l is extremely small. This is quite important. Measures of the Balmer discontinuity have proved very useful for spectral classification (BAI~BIEI~,CHALONGE,and VASS¥, 1935 ; 0HMAN, 1935 ; ()I~MANand IWANOWSKA, 1935; BARBIER and CHALONGE,1939, 1941 ; CHALONGEand DIVAN, 1952). WILHELM BECKER in a number of investigations has demonstrated the possibilities of threecolour photometry which includes one ultraviolet intensity strongly influenced by the Balmer discontinuity. Recent work along these lines include investigations by JOHNSON and MORGAN(1953) and by BARBIER (1952). BARBIER has shown empirically that it is possible to derive a quantity equal to the Balmer discontinuity from the intensities of the six-colour photometry of STEBBINS and WHITFORD (1945). For the purpose of spectral classification it is essential that the classification index in question has the following properties: (1) it must vary strongly with the Balmer discontinuity, although it is not necessary that it actually measures the discontinuity; (2) it should be uninfluenced by interstellar reddening. The use of interference filters makes it possible to select wavelength regions defining a Balmer discontinuity index relatively close to each other. They can be so chosen that interstellar absorption which follows the normal wavelength dependence does not influence the index. When the wavelengths are rather close together even a relatively marked change of the absorption law (such as occurs for instance, in the Orion region) will not lead to any marked variation of the index with the amount of interstellar absorption. [t was found (STROMOREN, 1951, ]952) that a classification index formed from the intensities at 3550 A (a UV filter), 4030 A (interference filter between H~ and He), and 4500 A is a satisfactory Balmer discontinuity index. The Balmer discontinuity index is defined as follows, c ~ 2.5 {2 log 1(4030) -- log •(3550) -- log 1(4500)) ~- const . . . . .

(2)

Comparing the definitions of the indices 1 and c, equations (1) and (2), we may note the extra factor 2 in c as compared to 1. This factor must be kept in mind when the photometric accuracy obtained in the measures of 1 and c is discussed. From what was known observationally, and also theoretically, regarding the variation of the strength of Hfl and of the Balmer discontinuity with effective temperature and gravity it was expected that the two indices l and c would yield satisfactory two-dimensional classification for B, A, and F stars. Measures made in November and December, 1951, with a photoelectric photometer attached to the 82-in. reflector of the McDonald Observatory showed that very accurate two-dimensional classification on the basis of measures of the Hfl index l, equation (1), and the Balmer discontinuity index c, equation (2), could actually be obtained for B, A, and F stars.

BENGT STROMGREN

1339

The investigation was continued in February and March, 1953, with the same instrument. The filter set had been slightly improved in t h a t noviol glass filters were used to cut out a weak ultraviolet transmission t h a t had affected the measures with the 4030 and 4500 filters. Also the Hfl interference filter was tilted somewhat to place its maximum transmission exactly at 4861/~. The following is a summary of the results obtained during the latter period on the calibration of the classification method for F stars. In the calibration programme sixty-seven F stars and a few A stars, G stars, and metallic line stars were observed. Most of the F stars in the Henry Draper Catalogue brighter than 5'.~5 between 5h and 16h right ascension and north of declination -- 10° were observed. The photoelectric photometry was carried out with a refrigerated 1P21 photomultiplier using a linear amplifier and a Brown recorder. The filter observations were arranged as follows. A sequence of seven deflections, 5000, Hfl, 4700, Hfl, 5000, Hfl, 4700 was taken to determine the Hfl index, the sky reading being determined separately for each filter. Seven deflections, similarly arranged, gave the Balmer discontinuity index. The total observing time for the determination of both indices averaged about 8 minutes. The interference filters, of course, reduce the star intensity considerably more than do standard glass filters. In comparison with standard three-colour photometry the reduction is about 3 magnitudes for the Hfl filters, and about 2 magnitudes for the Balmer discontinuity filters. During the observing period in question interference filter photometry on the F star programme and other programmes was carried out on thirteen nights. Extinction was determined on ten nights. The extinction proved to be entirely negligible for the Hfl index l, as was to be expected. For the Balmer discontinuity index c the mean extinction coefficient was found to be 0.m070. As the range in the value of sec z (z is the zenith distance) was a few tenths only for the great majority of the programme stars, the mean extinction coefficient was used for the reduction of the index c to the zenith. For both 1 and c night corrections, constant for each night, were determined and applied to reduce the results to a homogeneous system. Six standard stars were observed repeatedly to determine the night corrections. The average number of standard stars observed per night was 4.3. The mean of the night corrections without regard to sign was 0.m002 for the index l, i.e. quite small, and 0.m018 for the index c. Any variation in the extinction coefficient for c (reduction from air mass 1 to air mass 0) from night to night is, of course, absorbed in the night correction. With very few exceptions the programme stars have been observed on at least two nights. The average number of observations per star, excluding the standard stars, is 2.9. The accuracy of the photometric observations was determined from the differences in the index values obtained on different nights (after application of the night corrections). The probable error of one observation of the Hfl index 1 was found to be ± 0.~003, while a value of i 0.m008 was found for the probable error of one observation of the Balmer discontinuity index c. Recalling the factor 2 in the definition of c we see that the probable error is about 30 per cent higher in the c-photometry than in the/-photometry. This is undoubtedly due to the fact that the wavelengths defining the c-index are more widely separated than those defining the /-index and t h a t

134o

Two-dimensional

spectra[ classiiicatiou of F stars

they inchlde the ultraviolet, st) t h a t the c-index is m(>re influenced by variations i)l atmospheric transmission. 3.

(I()MPARISON

WITH

SPECTRAL

(~LASSIFICATION

BY

~V.

~V.

MOR(IAN

The results obtained were first compared with spectral classes and luminosity classes according to a recent, determination by W. W. MORGAN. T a b l e I gives the spectral classification and the values of l and c for t w e n t y F stars and for a few additional A and G stars. Table 1 HR 1017 1069 1135 1412 1543 2484 2693 2852 3188 3579 3616 3775 3873 3888 4031 4054 4112 4540 4883 4931 4983 5185 5435 5634 5933 6685

Sp. ¢¢ P e r 36 P e r 41 P e t ' 0~ Tau ~r3 O r i ~ Gem ~) C M a p Gem ~ Mon --~" U M a 0 UMa eLeo vUMa ~ Leo 40 Leo 36 U M a A fl V i r 31 C o r n 78 U M a [3 C o r n ~ ]30o ~ Boo 45 B o o ~+ S e t 89 H e r

F5 ib F 4 H~ F 5 rt A 7 HI F6 v F 5 HI F8 Ia F0 v G 2 Ib F5v F71v v F 6 IV G0H F2Iv F 0 H H[ F6 iv F8 v F8 v GO H~ F2 v G0 v F7 v A7 H~ F5 v F6 v F2 la

tiff index 1

Bahner

0'~?092 0" l I 0 0.106 0.207 0. 084 0'097 0"064 0"132 0"025 0. 098 0"080 0.072 0.03l 0.135 0 . 136 0. 096 0.064 0- 0 6 2 0"038 0 . 130 0" 0 6 0 0.092 0-204 0 . 104 0.088 0.091

0"784l 0"580 0"832 0.978 0' 434 0"536 0"681 0.650 0 . 351 0.512 0.439 0.477 0- 3 9 2 0 . 811 0.892 0- 4 8 6 0-368 0- 4 0 2 0' 364 0.600 0. 354 0.445 0' 989 0.494 0.435 0.99[

index c

The results given in Table 1 are shown graphically in Fig. 1. It is seen that there is a very close correspondence between MORGAN'S classification and the classification given by the indices 1 and c. Only two stars deviate, namely, H R 3579, which would be classified as of luminosity class IV from 1 and c, while MORGAN'S luminosity class, confirmed by the trigonometric parallax, is V, and r Boo, which would be classed as F6 from 1 and c instead of FT. It should be noted that the first-mentioned star is a close binary with AM = 2 m. With the help of Fig. 1 it can be estimated that the photometric accuracy of =]= 0':'003 (p.e.) in 1 and ~ {)':'008 (p.e.) in c should correspond to an accuracy of about 0.02 (p.e.) in spectral class derived from 1 and c. This conclusion is confirmed through the discussion of colour indices. 4.

COMPARISON

WITH

(]OLOUR

INDICES

One of the aims of the present investigation was to develop a m e t h o d that would yield accurate predicted colour indices for F stars. Accurate colour excesses could then be determined for a group of stars of higher space density, greater average distance from the galactic plane, and more even distribution than the group of B stars. The advantages in galactic research are obvious.

B E N G T STRSMGgEN

.........

eAT IH eA7 m

'I"'

.......

I . . . . .

,,it

........

1341

I ........

'l

.........

I ......

''

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15(

For

F~V

,4.= F~n

~sv

I0(

FS~tTr FSe~"

~la

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co.~z co~ czzh

L] 000

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I I I I I i h I I I I I I I I I I I I I I'l 900 $00 700

ILI

I I I I I L I , III bOO

I ~ I_l__Ll .~OO

I I III

I I I I III 400

I..Lll

c

Fig. I. T h e relation b e t w e e n t h e photoelectric classification indices l a n d c, a n d spectral classes a n d l u m i n o s i t y classes d e t e r m i n e d b y W. W. MOR~A.~*

n 300

1342

Two.dimensional spectral classification of F stars

In order to test the accuracy with which a colour index can be predicted from the Hfl index l and the Balmer discontinuity index c, photoelectric colour indices were measured for the stars for which 1 and c had been determined. With the exception of the stars of luminosity class I and a few of the stars of luminosity class I I the prog r a m m e stars are all so close t h a t interstellar reddening must be quite small. The measured values of l and c can therefore be compared with the measured colom" indices for the purpose of calibration and for a d e t e r m i n a t i o n of the accuracy with which the colour index can be predicted from l and c. Photoelectric colour indices, in HAROLD JOHNSON'S (B-V) system, have been determined for all the p r o g r a m m e stars b y Mr. PETER NAUR. The 13-in. reflector of the McDonald O b s e r v a t o r y was used for these determinations, made in F e b r u a r y and March, 1953. The definitive results are not yet available, however, and in the present investigation provisional colour indices, also in HAROLD JOHNSON'S (B-V) system, have been used. These were d e t e r m i n e d from the 82-in. interference filter measures themselves, use being made of the intensities at 4030 A, 4500 A, 4700 A, and 5000 A. For sixteen of the p r o g r a m m e stars v e r y accurate (B-V) colour indices were available according to measures at the McDonald O b s e r v a t o r y b y HARRIS and H a r o l d JOHNSON. The colour indices for the rest of the p r o g r a m m e stars were d e t e r m i n e d from the k n o w n colour indices, using the interference filter intensities, through a process of linear interpolation. F r o m the accuracy with which the colour indices for the sixteen s t a n d a r d stars were reproduced, and from the agreement between the results obtained on different nights it was concluded t h a t the resulting colour indices have a probable error of ~: 0~.~006 (mean of 2.3 observations). The true probable error m a y be somewhat greater t h a n this. Although the accuracy of NAUR'S colour indices will u n d o u b t e d l y be better, it was felt t h a t the provisional colour indices were sufficiently accurate for a significant test of the colour indices predicted from 1 and c. Fig. 2 shows the p r o g r a m m e stars in an (l-c) diagram, with the (B-V) colour index, in units of o n e - h u n d r e d t h of a m a g n i t u d e indicated above the dot corresponding to the star. (The absolute m a g n i t u d e is given below the dot ; see the following section.) (~olour indices are not given for the stars of luminosity class I and I I t h a t m a y be appreciably reddened b y interstellar absorption. Also, a few binaries with Am less t h a n 3 m have been omitted. I t is seen t h a t the correspondence between l and c, on the one hand, and (B-V), on the other, is quite close. The calibration and the q u a n t i t a t i v e test of the accuracy with which B-V can be predicted from 1 and c were carried out as follows. The stars were divided into two groups according to w h e t h e r the Hfi index l was smaller or larger t h a n 122, cf. Fig. 2. In the group with l < 122 stars with (B-V) < 0'~53 were included. B e y o n d this limit the strength of the h y d r o g e n criteria tends to be too small for accurate prediction of the colour from l and c. In this group t w e n t y - s e v e n stars with k n o w n l, c, and (B-V) were available. In the group with 1 > 122 the limit (B-V) > 0~-~28 was chosen, and fifteen stars were available. F o r each of the groups a linear expression for (B-V) in terms of 1 and c was assumed, and the coefficients d e t e r m i n e d from a least squares solution. F o r the group with l -< 122 the following expression was derived, B-V ~ 0m677 -- 1"760/ -- 0"164c .

.

.

.

.

(3)

This expression reproduced t h e (B-V) values with residuals t h a t gave a probable error of one difference, observed minus predicted colour index, equal to ± 0'."009.

BENGT

1343

STR()MGREN

''''''''l'''''''''l'''''''''l'''''''''l ZOO

--

.........

I

. . . . . . . . .

I I[

......

1"2

O.b

3e4me,~l,.

3,0 15C

~ met t

2"6

30 -,.o

30

2.2

2.o

~

~

14

2.4

~3

2"7

29~

2-5~ ]O

,'o

3 38 e40 3,

3q

_2.'0

4~2

3'5

I0¢

?o

•

2.0

-:5

-~?.o

3"8 41 4403 ~ 4.5 30

45

=6 ,o 47

.//16

42 6

7 38

4,'0 2"4 4 @29 2"~50 21 -~6

~5 39

43 56 57:'~'6 3$

48

&

-~o

DO0

....

: I I I I I I , ..... 900

I ......... 800

F i g . 2. T h e r e l a t i o n b e t w e e n t i l e indices and absolute magnitudes. MORGAN'S s p e c t r a l c l a s s i f i c a t i o n , BRAYTON (Ap. J., 8 1 , 187,

37

I ......... 700

I ......... bOO

I ..... 500

t L I I I I I ILl 400 C

p h o t o e l e c t r i c c l a s s i f i c a t i o n i n d i c e s l a n d c, a n d c o l o u r The latter were derived using trigonometric parallaxes, a n d t h e c a t a l o g u e b y ADAMS, J o Y , H U M A S O N , a n d 1935). Metallic line stars are indicated by met. l

I , #

300

1344

Two-dimensional spectral classification of F stars

For the group with l > 122 the range of l an
The probable error of the provisional colour indices utilized was estimated above to be ± 0':1006. Combining the probable errors for the observed and the predicted value of (B-V), the probable error of one difference is found to be ± 0+007. This is somewhat less than the value determined above, or ± 0'~.~009, indicating that the latter value may be somewhat increased by cosmical scatter in the relation between l, c, and (B-V). However, the explanation may also be that the accuracy of the provisional colour indices is somewhat less than that indicated by the probable error ()~:~006. When the definitive colour indices referred to above are available it may become possible to settle the question. It is safe to conclude that the cosmical scatter in the relation between l, c, and (B-V) is quite small, corresponding to a probable error considerably less than onehundredth of a magnitude. This is in agreement with a conclusion reached by the author (STROMGREN, 1952) from a previous discussion of a smaller material. Also, we conclude that one observation by the (l-c) method yields a colour excess for F stars with a probable error nearly equal to one-hundredth of a magnitude. The photometric probable error of a predicted B-V value (one observation) of -t- 0+006 corresponds to a probable error of ± 0.02 of a spectral class, a value in agreement with t h a t estimated from Fig. 1. From the theoretical point of view it is understandable that the cosmical scatter in the relation between l, c, and (B-V) is small for the F stars. The relation is quite insensitive to possible variations from star to star in the relative abundances of the elements. This is an advantage characteristic of a method of classification based on hydrogen criteria alone. In this connection the high classification accuracy obtained by D. CHALONGEand LUCIENNE DIVA~~ (1952), using the Barbier-Chalonge method based on photographic spectrophotomctry of the region around the Bahner discontinuity, should be mentioned. It may be noticed that the three metallic line stars measured fall outside the region of the (l-c) diagram in question, and that stars of this type therefore do not give rise to any difficulties here. 5. COMPARISON WITH ABSOLUTE MAGNITUDES FROM TRIGONOMETRIC PARALLAXES AND SPECTROSCOPIC DETERMINATIONS

We shall now turn to the question of the determination of visual absolute magnitudes M~ from the indices I and c. The discussion is based upon values of M~ derived from trigonometric parallaxes, and from MORGAN'S spectral classification. Weighted means between the trigonometric and spectroscopic values were adopted, the relative weights depending, of course, upon the size of the parallax. The relation between l, c, and M~ is shown in Fig. 2.

BENGT STROMGREN

1345

Again, for a quantitative discussion of the relation the material was divided into two groups with 1 ~ 122, and linear expressions for M v in terms of l and c were assumed. For 1 < 122 it was found that the following linear relation reproduces the observed M,, values within their observational errors, namely, M v = 8'~'37 ~- 39.2/ -- 18.2c.

.

.

.

.

(4)

The residuals gave a probable error for one difference, observed minus predicted absolute magnitude My, of -4- 0~.'25. This corresponds to the probable error of the adopted M v values, and there is then no evidence of cosmical scatter in the relation between M y , l, and c. From equation (4) and the known probable errors of one observation of 1 and c it follows that the photometric probable error of the My value predicted from l and c (one observation) is -4-0.m18. The corresponding probable error for an average programme star (2.3 observations, cf. above)is ± 0':~12. For the group with 1 > 122 a linear expression is also satisfactory, but the coefficients are not very well determined because the range of variation in I and c is small. The observed values of M~ are reproduced with residuals corresponding to a probable error of one difference, observed minus predicted, equal to -4- 0'.~4. The somewhat larger value is connected with the fact that the observed values of M e , because of the smaller trigonometric parallaxes, are less accurate here. We may note in passing that the position of the metallic line stars in the (l-c) diagram if taken at its face value corresponds to smaller luminosities than the main sequence stars of the same Balmer discontinuity. The photometric probable error of the M v value predicted from one (l-c) observation is so small that more extended observations are required to test the ultimate possibilities of the method. Although the results of the present calibration may be satisfactory for some purposes, a more accurate calibration based on trigonometric parallaxes (and possibly proper motions and radial velocities) for a much larger number of programme stars would be very desirable. Also, applications of the method to stars in galactic clusters might lead to valuable results. It is outside the scope of the present paper to discuss applications of the classification method. It may be mentioned, however, that Mr. DANIEL HARRIS and the author have tackled the problem of determining the interstellar reddening as a function of distance in the direction of the galactic pole from colour excesses of F stars. Mr. NAUR, Mr. HARRIS, and the author have made preparations (through determination of rough spectral types and photoelectric colour indices of stars down to 11 m) for a similar study in the region of the Taurus nebula. A long range programme of colour excess determinations for a large number of F stars nearer than 400 parsecs, for the purpose of charting interstellar absorption in this region of the galactic system, is contemplated. I am grateful to the Board of the Observatoire de Haute-Provence for the opportunity given to Mr. GYLDENKARNEto use the 32-in. reflector at an early stage of the investigation of the possibilities of the interference filter method of classification. I wish to express sincere thanks to Mr. HARRIS and Mr. NAUR for their valuable help during the observational work with the McDonald 82-in. reflector, and to Mr. PEROLOF LINDBLAD for his effective assistance in the reductions. Mr. A. F. TURNER, Bausch & Lomb Optical Co., put at my disposal the excellent Hfl filter used. I am particularly indebted to Mr. W. W. MORGANfor communicating results of his spectral

1346

L u m i n o s i t i e s of t h e B s t a r s f r o m spectroscopic m e a s u r e m c H t s

classification work in advance of publication, and for many helpful (liscussiolls throughout this investigation.

|{EFEREN( ES I~ARBIER, D., CHALON/'VL D. a n d VASSV:~, E . . BARBIE~, D. a n d CHALONCE, D . . . . . . BARBIER, D . . . . . . . . . CHALONGE, e . a n d LUCIENNE DIVAN JOHNSON, H. L. a n d MORGAN, W . W . 0HMAI~, Y . . . . . . . . . . . OHMAlU, Y. a n d IWANOWSKA, W . . . STEBBINS, J. a n d WHITFORD, A. E . . STROMGREN, B . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . .

1935 1939 1941 1952 1952 1953 1935 1935 1945 ] 950 1951 1952

J. Phys. Radiltm, 6, 137. A~n. Astrophys., 2, 154. Ann. Astrophys., 4, 30. Ann. Astrophys., 16, I l:~. Ann. Astrophys., 15, 201. Ap. J., 117, 313. Stockholms Ob.~'. Aun., 12, No. I. Stockholms Obs. Medd., 21. Ap. J., 102, 318. Transactions of the l~ter~atioaal Astro~.. Union, vol. 7, 404. Astron. J., 56, 142. Astron. J., 57, 200.

Luminosities of the B Stars from Spectroscopic Measurements R. M. PETRIE D o m i n i o n A s t r o p h y s i c a l O b s e r v a t o r y , Victoria, B.(~. SUMMARY T h e p r o b l e m of a s s i g n i n g d i s t a n c e s to t h e B s t a r s is reviewed a n d t h e d e v e l o p m e n t of t h e spectroscopic m e t h o d briefly described. I t is e s t a b l i s h e d t h a t t h e s t r e n g t h o f h y d r o g e n l i n e - a b s o r p t i o n g i v e s d i s t a n c e s b y i n d i e a t i n g t r u e l u m i n o s i t y . T h e Victoria w o r k is a n e w calibration o f w h i c h t h e chief f e a t u r e s are : (a) t h e h y d r o g e n a b s o r p t i o n is m e a s u r e d b y s p e c t r o p h o t o m e t r i e m e t h o d s ; a n d (b) c a l i b r a t i n g s t a r s are d r a w n solely f r o m galactic d u s t e r s , visual binaries, a n d eclipsing binaries. A correlation is f o u n d b e t w e e n a b s o r p t i o n W a n d a b s o l u t e v i s u a l m a g n i t u d e M o f t h e f o r m M = -- 10.96 × 10 -0"145W. T h e s y s t e m so defined agrees w i t h t h a t of t h e t r i g o n o m e t r i c p a r a l l a x e s , w i t h eclipsing b i n a r y luminosities, a n d w i t h the m o v i n g - c l u s t e r l u m i n o s i t i e s of t h e 8corpio-Centaurus s t r e a m . Some a p p l i c a t i o n s are i n d i c a t e d a n d a first list of spectroscopic luminosities is g i v e n for 124 stars.

1. INTRODUCTION THE great intrinsic brightness of stars of spectral class B renders them of special interest and value to astronomers. Because of their high luminosities we may observe them at great distances from the Sun and indeed we depend upon them for explorations of the galaxy, to determine its motions and dynamical properties, and to probe the interstellar clouds of gas and dust. In addition, the B stars are, generally speaking, among the most massive of the stellar population ; this fact coupled with the enormous output of energy makes them of special interest to students of stellar structure and stellar evolution. It is obvious that our investigations will be hampered unless we have a reliable means of assigning distances to the B stars or, alternatively, a method of determining their luminosities. The~ great number of researches during the past three decades testifies to the importance of the problem and is also a tribute to its difficulty. We require for many explorations a relatively accurate assignment of absolute magnitude.