<figure><img src="image_130.png" /><figcaption>FIG. 1.—Model fitted to the available hydrogen Balmer lines in our time-averaged optical spectrum of KPD 2109+4401.</figcaption></figure>

2109+4401. This target star was selected from the work of Saffer & Liebert (1998), which is still in progress, and we take the opportunity here to report on these time-averaged spectroscopic observations as well as on the photometric variability itself. We also report on the current status of our ongoing search for more of these pulsators. We note that our discovery brings the number of EC 14026 stars known in the public domain to a total of seven. We learned recently of the existence of an exciting eclipsing binary, PG 1336−018, whose sdB component is an EC 14026 pulsator (Kilkenny et al. 1998). We are aware that a handful of additional pulsators of the same type have been discovered and are being studied in detail by the South African group. We are also aware that their search has been extended to the northern hemisphere. After this work was essentially completed, we received a preprint from Koen (1998) describing his independent discovery of luminosity variations in KPD 2109+4401.

## 2. OBSERVATIONS

KPD 2109+4401 is a relatively bright object (V = 13.38) that was first recognized as an sdB star in the Kitt Peak–Downes survey of UV-excess objects in the galactic plane (Downes 1986). We carried out follow-up high signal-to-noise ratio optical spectrophotometry with the 2.3 m telescope at the Steward Observatory Kitt Peak Station. This was before we knew of the variability of the object. The experimental setup, reduction procedure, and the model atmosphere and synthetic spectrum analysis were the same as those described in Saffer et al. (1994), to which the reader is referred for details. Our optical spectrum covers the wavelength range 3650–5200 Å at ~6 Å resolution

<figure><img src="image_131.png" /><figcaption>FIG. 2.—First part of the light curve of KPD 2109+4401, observed on 1997 September 29 with the Mount Bigelow 1.6 m telescope, expressed in residual amplitude in units of millimagnitude. The light curve is continuous, starting at the upper left, and each point represents a sampling time of 10 s. The segment shown here corresponds to the first 10,000 s of a total light curve 25,030 s long.</figcaption></figure>

and required an integration time of 600 s. Given the relatively short periods and the small amplitudes of the pulsations (see below), we consider that this spectrum represents a meaningful time-averaged spectrum of KPD 2109+4401.

Figure 1 shows our best (in a least-squares sense) fit to the available hydrogen Balmer lines. The derived atmospheric parameters are $T_{eff} = 31,200$ K and $log g = 5.84$, with formal 1 σ errors of 725 K and 0.16 dex, respectively. These parameters are representative of the six previously known EC 14026 stars, except that KPD 2109+4401 appears to be somewhat, but significantly, cooler than the others. They also place KPD 2109+4401 well within the theoretical instability strip discussed by Charpinet et al. (1997). We note that, contrary to what was found in the first four known EC 14026 stars, our optical spectrum of KPD 2109+4401 does not show any sign of contamination by a cooler companion. Our model atmosphere analysis is thus free of the additional complications introduced by the procedure used to remove the contribution of a cool companion of uncertain spectral type. From that point of view, KPD 2109+4401 must be considered as a “single” star, very much like PG 1047+003, at least at visual wavelengths.

KPD 2109+4401 was observed in white-light “fast” photometric mode during 25,030 s at the Mount Bigelow 1.6 m telescope on 1997 September 29 under excellent conditions. The photometric observations were gathered with LAPOUNE, the portable Montréal three-channel photometer. We refer the interested reader to Billères et al. (1997) for details on the observational procedure. We show, in Figure 2, a typical segment of the sky-subtracted, extinction-corrected light curve. It is expressed in terms of residual amplitude in units of millimagnitude, and each plotted point corresponds to a sampling time of 10 s. The light curve clearly reveals the presence of multiperiodic brightness modulations. The variations appear to be dominated by a pseudoperiod near 200 s, but it is obvious
No. 1, 1998

p-MODE PULSATIONS IN KPD 2109+4401

L77

<figure><img src="image_132.png" /><figcaption>FIG. 3.—Low-frequency part of the Fourier (amplitude) spectrum of the full light curve of KPD 2109+4401 observed on 1997 September 29 with the Mount Bigelow 1.6 m telescope. The amplitude axis is expressed in terms of the variations (in units of millimagnitude) about the mean intensity of the star. Five periodicities are clearly seen.</figcaption></figure>

that several periodicities are involved. For instance, peak-to-peak variations can sometimes reach close to 60 mmag, but, on other occasions, partial destructive interference between modes does occur.

A standard Fourier analysis of the light curve reveals more quantitatively its detailed structure. This is illustrated in Figure 3, which shows the most interesting part of the Fourier (amplitude) spectrum of the full light curve. Our observations reveal that all the photometric activity is concentrated in a rather narrow range of frequency, with no detectable signal outside that range. In particular, the spectrum in the region from 9 mHz out to the Nyquist frequency (50 mHz) is entirely consistent with noise. The noise level in that spectral region is less than ~0.3 mmag.

We can distinguish five significant components in the Fourier spectrum of the light curve of KPD 2109+4401. The dominant peak at 5.094 mHz (196.3 s) has an amplitude $\Delta m \approx 8.6$ mmag. There is an adjacent peak, barely resolved from the main peak, located at 5.040 mHz (198.4 s) and showing an amplitude $\Delta m \approx 5.4$ mmag. Those are mostly responsible for the pseudoperiod of ~200 s seen in the light curve itself. Other peaks in the Fourier spectrum are found at 5.223, 5.420, and 5.479 mHz corresponding to periods 191.5, 184.5, and 182.5 s, and amplitudes $\Delta m \approx 3.6, 2.3, \text{and } 6.0$ mmag. These results are in excellent agreement with the independent study of Koen (1998), who finds the same five periodicities in his data. Koen (1998) was able to reach a much higher temporal resolution than that shown here and found that there is fine structure in at least the 196.3 and 184.5 s peaks, possibly caused by rotational splitting.

A comparison of the observed periods with periods computed from models of sdB stars having atmospheric parameters comparable to those of KPD 2109+4401 shows unambiguously that, as in the case of the other EC 14026 stars, the luminosity variations must be identified with low-order, low degree radial and nonradial (p and f) pulsation modes. Details on the model building and pulsation codes will be reported elsewhere, but the tools we used are the same as in Charpinet et al. (1997) and Fontaine et al. (1998). We note that Figures 2 and 3 of Fontaine et al. (1998) are particularly useful for the kind of qualitative comparison needed in the present context.

<table><caption>TABLE 1
CLASS PROPERTIES OF EC 14026 STARS</caption><thead><tr><th>Name</th><th><i>T</i><sub>eff</sub><br>(K)</th><th>log <i>g</i></th><th><i>P</i><br>(s)</th><th>Δ<i>m</i><br>(mmag)</th><th>Reference</th></tr></thead><tbody><tr><td>EC 14026-2647</td><td>34,700</td><td>6.10</td><td>134-144</td><td>4-12</td><td>1</td></tr><tr><td>PB 8783</td><td>35,700</td><td>5.54</td><td>120-134</td><td>1-9</td><td>2</td></tr><tr><td>EC 10228-0905</td><td>33,500</td><td>6.00</td><td>139-152</td><td>4-14</td><td>3</td></tr><tr><td>EC 20117-4014</td><td>34,800</td><td>5.87</td><td>137-159</td><td>1-4</td><td>4</td></tr><tr><td>PG 1047+003</td><td>34,370</td><td>5.70</td><td>104-162</td><td>2-9</td><td>5</td></tr><tr><td></td><td>35,000</td><td>5.90</td><td>...</td><td>...</td><td>6</td></tr><tr><td>PG 1336-018</td><td>33,000</td><td>5.70</td><td>141-184</td><td>5-10</td><td>7</td></tr><tr><td>KPD 2109+4401</td><td>31,200</td><td>5.84</td><td>182-198</td><td>2-9</td><td>8</td></tr></tbody></table>

REFERENCES.—(1) Kilkenny et al. 1997; (2) Koen et al. 1997; (3) Stobie et al. 1997; (4) O'Donoghue et al. 1997; (5) Billères et al. 1997; (6) O'Donoghue et al. 1998; (7) Kilkenny et al. 1998; (8) this work.

### 3. DISCUSSION AND CONCLUSION

It is interesting to make some comparisons with the known EC 14026 pulsators. Table 1 summarizes the class properties of these stars. It is obvious that KPD 2109+4401 is a very similar object and, thus, a genuine member of the EC 14026 family. We note that, on theoretical grounds (see Fontaine et al. 1998), it is expected that a cooler EC 14026 star (of given surface gravity) should show the larger excited periods. It is thus perhaps significant that KPD 2109+4401 shows both the smaller effective temperature and the larger excited periods of the known EC 14026 stars.

We have plotted the positions of the seven publicly known sdB pulsators in the $T_{\text{eff}}$–log g plane (Fig. 4). Note, in particular, how the empirical instability strip has widened as a result of the addition of KPD 2109+4401 in the class. Superimposed in the same diagram is the updated distribution of nonvariable stars found in our ongoing survey. A total of 52 sdB stars have now been found “constant,” in the sense that we rule out white light brightness variations with amplitudes larger than ~1 mmag in the 20–1200 s period window for these objects. Note

<figure><img src="image_133.png" /><figcaption>FIG. 4.—Current distribution in the $T_{\text{eff}}$–log g diagram of the sdB stars observed in our ongoing survey. The small filled circles represent the 52 stars found nonvariable. The location of KPD 2109+4401 as well as those of the six previously known EC 14026 stars are indicated by a circled dot. A number identifies each of the pulsators.</figcaption></figure>
that, as the properties of EC 14026 stars became better known, we relaxed somewhat our search criteria. In particular, given the expected pulsation periods, we now spend no more than 1–2 hr of 1.6 m telescope time on unpromising targets, depending on the brightness and observing conditions.

Figure 4 allows us to reiterate an important point made earlier by Billères et al. (1997): with due caution associated with the uncertainties in the individual determinations of the atmospheric parameters of sdB stars, coupled with the fact that estimates of these parameters *do* suffer from systematic differences from one study to another (see Wesemael et al. 1997), there is a clear hint that both pulsators and nonpulsators may coexist in the same part of the *T*<sub>eff</sub>–log g diagram. This suggests that an additional mechanism may operate at widely different

efficiencies in two sdB stars with, otherwise, similar values of *T*<sub>eff</sub> and log g. Weak stellar winds represent a very likely possibility (Charpinet et al. 1997; Fontaine & Chayer 1997; and references therein).

It is a real pleasure for us to acknowledge the stimulating discussions on EC 14026 stars that we had with Darragh O'Donoghue, Dave Kilkenny, Chris Koen, and Bob Stobie. We also wish to thank the directors, staff, and members of Time Allocation Committees of the CFHT, Michigan-Dartmouth-MIT, Mont Mégantic, and Steward Observatories. This work was supported in part by the NSREC of Canada and by the fund FCAR (Québec).

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# The EC14026 Stars PG1219+534, PG0911+456: Successes and New Challenges for the Fe Driving Mechanism

C. Koen<fnref n="1" />,<fnref n="2" />, D. O'Donoghue<fnref n="1" />, D.L. Pollacco<fnref n="3" />, S. Charpinet<fnref n="4" />

## ABSTRACT.

We report the discovery of short period oscillations in the sdB stars PG0911+456 and PG1219+534, establishing them as members of the EC14026 stars, a recently discovered class of sdB pulsators. The atmospheric parameters for both stars are also determined. The results are: periods of (148.8, 143.7, 133.5, 128.1 s), T<sub>eff</sub> = 32800 K, log g = 5.76 for PG1219+534, and periods of (165.7, 161.5, 155.8 s), T<sub>eff</sub> = 31900 K, and log g = 5.82 for PG0911+456.

The pulsation periods of the 11 well-studied EC14026 stars are compared with envelope models from Charpinet et al. (1998). With the exception of PB8783, the longest period in each star is close to the periods of the lowest order modes in the models. Arguments are given that this is also true for PB8783, involving an increase in its surface gravity over the value derived in an earlier paper. “Correcting” the observed periods for the effects of differing surface gravity from star to star, comparison is made with pulsation models of constant surface gravity and varying effective temperature. Reasonable agreement in the width of the instability strip is demonstrated, as well as properties of its cool boundary. Overall, the class properties of the EC14026 stars are in impressive agreement with the Fe driving mechanism.

PG1219+534 has an atmosphere much richer in He than most sdB stars and all EC14026 pulsators seen so far. As the Fe driving mechanism operates if sufficient heavy elements accumulate at the appropriate layer in the stellar envelope, the mechanisms (gravitational settling, radiative levitation, weak stellar winds) which give rise to this must simultaneously be able to provide large differences in the abundance of photospheric He.

**Key words:** stars : variable, oscillating – individual: PG1219+534, PG0911+456

<footnote n="1">¹South African Astronomical Observatory, PO Box 9, Observatory 7935, Cape, South Africa</footnote>

<footnote n="2">²Dept. of Astronomy, University of Texas at Austin, Austin, TX 78712, USA</footnote>

<footnote n="3">³Isaac Newton Group, Santa Cruz de La Palma, Tenerife 38780, Canary Islands, Spain</footnote>

<footnote n="4">⁴Département de Physique, Université de Montréal, C.P. 6128, Succ. Centre-Ville, Montréal, Québec, Canada, H3C 3J7</footnote>