COMMISSION 27 OF THE I. A. U.
                   INFORMATION BULLETIN ON VARIABLE STARS
                                Number 2462

                                                      Konkoly Observatory
                                                      Budapest
                                                      20 January 1984      
                                                      HU ISSN 0374 - 0676


                          ON THE SX Phe-TYPE STARS


  About ten years ago one of the authors (Frolov, 1974) turned his attention 
to the physical peculiarities of Delta Scuti stars having short periods
(<0.1d) and large amplitudes (>0.3m). It was noticed that besides their specific
position on the period - amplitude diagram, they are totally lacking in
open clusters unlike usual Delta Scuti variables, and they often show metal
deficiency. Moreover, it was realized that several Population II objects
may also exist among Delta Scuti stars of long-periodic group, e.g. XX Cyg
having high space velocity.

  After lengthy discussion in the astronomical literature (on Delta Scuti
stars being Population I objects excluding only SX Phe itself) it would seem
that it is now agreed that there are several low mass low metallicity objects
among these stars. The majority of these stars indeed have P<0.1d and large
pulsational amplitudes. All the doubts have disappeared with the discovery
of two such variables NJL 220 (Niss, 1981) and NJL 79 (Jorgensen, 1982) in
the globular cluster omega Cen; both have nearly the same physical parameters as
SX Phe (P = 0.055d) itself: their amplitudes are about 0.5m B, periods are 0.046d
and 0.063d, and absolute magnitudes are +2.9 and +3.0. Even if these two stars
are not physical members of omega Cen, they have z-coordinates large enough 
(about 1 kpc) to be the objects of the galactic halo.

  In the paper on Delta Scuti stars in open clusters (Frolov and Irkaev,
1982) we have mentioned XX Cyg and RS Gru as being typical for globular 
clusters with different metal abundances on the basis of their low metal 
abundances or high space motion. XX Cyg and BDS 1269A = VW Ari with periods 0.135d
and 0.149d, respectively were mentioned by Breger (1979) as probable objects
of Population II. In reality, such objects among long-periodic Delta Scuti
stars can be more numerous due to faintness of many "Dwarf Cepheids" and to
usual lack of such information such as v_r and metal abundance. The only source
of information is the star's z-coordinate. In this way we can distinguish 4
more new Population II objects among long-periodic Delta Scuti stars having
z-coordinates exceeding 1 kpc: UW CVn (0.146d), MQ Pav (0.168d), LZ Her (0.199d)
and V 934 Oph (0.206d). Taking into account their periods, we adopted absolute
magnitudes equal to +2 or to +1; interstellar absorption was calculated




                              Table I

Star                     Period         Amplitude (V)     Criterion of Pop. II
SX Phe                   0.055d         0.5m var(0.37-0.73)     1,2
CY Aqr                   0.061          0.73                    1
DY Peg                   0.073          0.61                    1,2
KZ Hya = HD 94033        0.0595         0.80                    1,?
BL Cam = GD 428          0.039          0.30                    1,?
NJL 220                  0.046        >=0.5 B                   3, omega Cen
NJL 79                   0.063          0.48 B                  3, omega Cen
SU Crt = HD 100363       0.055          0.03                    1,2
XX Cyg                   0.135          0.90                    1,2
VW Ari = BDS 1269A       0.149          0.12                    2
BS Tuc = HD 6870         0.065          0.015                   2
RS Gru                   0.147          0.56                    1
UW CVn                   0.146          0.5 pg                  3
V 934 Oph                0.206          0.4 pg                  3
LZ Her                   0.199          0.5 pg                  3
MQ Pav                   0.168          0.5 pg                  3
 
according to Sharov (1963).
   We are already aware of seven Population II objects among long-periodic
Delta Scuti stars and nine among short-periodic ones including two stars in
omega Cen. Possibly the name "SX Phe-type stars" is not the most appropriate 
because about half of these variables have much longer periods than the prototype.
Possibly the situation with SX Phe stars is the same as for W Vir stars.
In both cases the Population II objects have been separated from the majority
of Population I stars and in both cases the period distribution is bimodal.
   In Table I we include all the Population II SX Phe-type objects known up
to date. In addition to the name of the star, its period, and light amplitude,
we show in the last column the criterion of Population II: 1 - high space 
motion; 2 - low metallicity; 3 - z-coordinate exceeds 1 kpc; "omega Cen" 
indicates that the star (NJL 79 and NJL 220) is in the globular cluster.




         M.S. FROLOV                   B.N. IRKAEV

     Astronomical Council of the       Dushanbe Astrophysical Institute
     USSR Academy of Sciences          of Tadjik Academy of Sciences
     Moscow, USSR                      Dushanbe, USSR




References:

Breger, M. 1979, Publ. Astron, Soc. Pacif., 91, 5. [BIBCODE 1979PASP...91....5B ]
Frolov, M.S. 1974, Sci. Inform. Astron. Council USSR Acad. Sci., No. 33, 29.
Frolov, M.S. and Irkaev, B.N. 1982, I.B.V.S., No. 2249.
Jorgensen, H.E. 1982, Astron. Astrophys., 108, 99. [BIBCODE 1982A&A...108...99J ]
Niss, B. 1981, Astron. Astrophys., 98, 415 [BIBCODE 1981A&A....98..415N ]
Sharov, A.S. 1963, Russian Astron. J., 40, 900. [BIBCODE 1963AZh....40..900S ]