PER 53 Persei stars

- Non-radial g-mode pulsating stars of spectral type 09-B5 showing line profile variations with periods ranging from ff! 16 to not recognized within the GCVS

53 Persei3 stars surround the instability zone of the ft Cephei stars on the HR diagram with spectral types that range from 09 through B5. Suggestions have been made to add the 53 Persei star group to the variable stars observed in the range B3-B8 and called "mid B variables." Eventually, it was decided to separate the two groups and to classify the mid-B variables as "slowly pulsating B stars."

The concept of 53 Persei variables was introduced in 1979 for 08-B5 stars that show variability with periods

'Perseus, ancient hero whose deed was to kill Medusa, a Gorgon, and eventually rescue the princess Andromeda from the sea monster Cetus.

Table 4.1. Pulsating variable stars arranged in alphabetical order by designation

Variable

53 Persei a Cygni ß Cep

BL Boo Cepheids

W Virginis

S Cepheids S Scuti y Doradus

Slow irregular variables

A Bootis

Mira Maia mid B variables

PV Telescopii RPHS

RR Lyrae

RV Tauri

Semiregular variables

Designation (and subclasses)

53 Per 09-B5 non-radial pulsating stars ACYG Be-Ae (emission) pulsating supergiants BCEP dossical ft Cephei stars BCEPS short-period ft Cephei stars BLBOO anomalous Cepheids CEP radially pulsating F lb-ll stars

CEP(B) double mode pulsators CW (two subclasses indicated below)

CWA population II, period > 8d CWB population I, period < 8d DCEP classical Cepheids, population I DCEP(S) classical Cepheids, with overtone DSCT A0-F5III/V pulsating stars DSCTC low-amplitude S Scuti stars GDOR early-type F dwarfs showing multiple periods L (two subclasses indicated below) LB late-type giants LC latetype supergiants LBOO "p", non-magnetic, A-F, population I dwarfs

M long-period late-type giants stars predicted to exist but none have been found Mid B B3-B6 stars; periods » 1-3 days; amplitudes of up to on 2 PVTEL helium supergiant Bp stars RPHS Very rapidly pulsating hot subdwarf B stars

(EC 14026 stars) RR (three subclasses indicated below)

RR(B) double-mode RR Lyrae stars RRAB RR Lyrae stars with asymmetric light cuiw RRC RR Lyrae stars with symmetric light curves RV (two subclasses indicated below)

RVa radially pulsating supergiants with constat mean magnitude _

RVb radially pulsating supergiants with variable mean magnitude SR (five subclasses indicated below) SRA

SRB M, C, S, Me, Ce, Se giants/poody defined periods

SRC M, C, S or Me, Ce, Se supergiants SRD F, G, and K giants/supergiants SRS semiregular pulsating red giants with short periods

SX Phoenicis SXPHE population II pulsating subdwarfs

UU Herculis * UUHer high-latitude F supergiants

ZZ Ceti stars ZZ (three subclasses indicated below)

ZZA hydrogen pulsating white dwarfs ZZB helium pulsating white dwarfs ZZO showing He II and C IV absorption lines

* Indicates a designation found within the literature but not recognized within the GCVS.

ciaure 4.1» curve

L variable star. 53

Z Cycle phase is indicated along the horizontal axis. Doto p,ovided by the HIPPARCOS mission. Used with permission

on the order of 24 hours. This is too long and too unstable to be associated with fi Cephei type variability, and eventually the 53 Persei variable stars were classified as non-radial pulsating stars so it would be impossible to associate them with the radially pulsating f) Cephei type variables (Figure 4.1).

Although 53 Per stars have long been suspected as non-radial pulsating stars, it is only very recently that pulsation instability within B-type stars is becoming understood. One of the characteristics of pulsation that recent studies are beginning to show is that pulsational instability is sensitive to fine details associated with metal opacities. Astronomers refer to all elements heavier than helium as metals.

Suggestions have also been made that the periodically variable B[e] stars may simply be 53 Per stars experiencing rapid rotation. However, this suggestion is usually considered to be wrong since 53 Per and B[e] stars are observationally distinct. Also, most B[e] variables have spectral types in the range which fall outside the instability domain for 53 Per stars.

This all suggests two different variability mechanisms for 53 Per and B[e] stars. In spite of many uncertainties, it seems very probable that the k-mechanism, due to metal opacity, is responsible for driving a selective set of high-order, low-degree g-modes within 53 Persei stars. We have no idea why only modes within a narrow period range are excited and why rotation damps these modes. It is of great importance to determine the boundaries of the 53 Persei strip more accurately by searching for these stars in open clusters. The mere fact that B[e] stars are so common among low-metal systems, relative to systems with normal metal content, is impossible to explain if the driving mechanism is the same as in 53 Persei and f) Cep stars, irrespective of anything else. In any case, there is ample evidence to justify the view that periodic variations are due to rotation and pulsation.

Because of their small amplitudes, these stars are best studied using photometric instruments such as CCDs or photometers.

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