PG0014 - A key sdBV pulsator

PG0014 - A key sdBV pulsator

PI: Steven Kawaler


Among the pulsating sdB stars (in particular the shorter period EC14026 stars) PG0014 stands out as one of the richest pulsators. Aside from PG1605, it shows the largest number of nonradial modes in the entire class, with at least 16 present in the discovery paper in the period range from 80 to 170 seconds (Brassard et al. 2001, herefter BFBCLS). This mode density poses a fundamental challenge to the theory of nonradial pulsations in these stars, as there are not enough radial and nonradial modes with low l (l<3) to account for all 16 without rotational splitting. One proposed solution to the large number of modes involves invoking high degree (l=3, 4) modes (BFBCLS). A more recent suggestion by Kawaler & Hostler (2003; hereafter KH) invokes evolutionary models of sdB stars, which show rapid internal rotation. With these models, the rich mode density would result from the presence of a few triplets (l=1) modes, quintuplets (l=2) and radial (l=0) modes.

The only available, published observations of PG0014 are those of BFBCLS, which are single site data from CFHT (5 runs on consecutive nights in 1998, each of three hours or less). These data show that the oscillation spectrum is indeed complex. Because of the time sampling of the data, they suffer from severe 1 cycle per day aliases. Extended longitude coverage of the WET will be needed to determine, uanmbiguously, the oscillation frequencies present in this star. Once that has been achieved, we can critically examine the theoretical models for these stars without concern for the reality of modes caused by diurnal aliasing.

In particular, we should be able to determine if high l (l >= 3) modes are required to match the observed oscillation frequencies or, if the rich mode spectrum is the result of rotational splitting of lower order modes by a rapidly rotating core. In either case, the rich observed mode spectrum of this star, as can be revealed by WET, will provide valuable input into modeling the structure of this important class of evolved star.


The Star

PG0014 was found to be a pulsating sdB star by the Montreal group, who followed up with several nights at the CFHT in 1998. The results of their 1998 runs have been published (Brassard et al. 2001, hereafter BFBCLS). This star undergoes pulsation in at least 16 modes, with periods ranging from 80 to 170 seconds. PG0014 itself is a 'typical' EC14026 star, with an effective temperature of 33,310K and log g = 5.8. The richess of its pulsations, however, (second only to PG 1605) make it a star of vital interest to the study of pulsating sdB stars and, by extension, the entire hot horizontal branch phenomenon. To fully exploit this star, we will need a global campaign to resolve the real pulsations from the inevitable 1 cycle/day aliases of single-site data.

PG 0014 is a rich pulsator that contains many oscillation frequencies. The general point that BFBCLS make is that there are too many modes than can be explained (using m=0 modes) with just l=0,1, and 2. They present a plausible fit to the modes they list with a model that includes l as high as 4. Rather than considering this as a precise fit (given the uncertainties in frequency identifications as described below) it is a sufficient demonstration that the mode density is, no doubt, higher than can be explained with the available low l (l < 3) m=0 modes. Within the assumption that higher l modes are present, a WET run that produces a more secure set of peak identifications can then be used for a precise model fit if desired.

Another possibility, explored by Kawaler & Hostler (2003, KH) is that PG0014, and sdB stars in general, have a rapidly rotating core. K&H have modeled the internal evolution of stars from the ZAMS to the EHB, including rotation, and show that the inner 0.3Mo should be rotating at rates between 15 and 1000 times (and more) faster than the surface. Their results are consistent with similar calculations performed for cooler horizontal branch stars, such as Sills & Pinsonneault (2000).

Such severe differential rotation can produce large rotational splitting in the observed modes (i.e. Kawaler, Sekii & Gough 1999). Though most of the rotational splitting of a mode is determined in the outer layers, the very rapid rotation in the core can still produce a much larger rotational splitting for a mode than would be indicated by the (slow) surface rotation velocity. This may be an explanation for the large splitting seen in PG1605 (Kawaler 1998) compared with the surface rotation velocity (Heber et al. 1999). Such an effect could also explain the somewhat large apparent rotational splittings seen in Feige 48 (Reed et al. 2003).

Sample results of the models of K&H were presented at the Keele EHB Workshop in June 2003. If K&H are correct, then the rich mode spectrum seen in PG0014 could be the result of only a few l=1 and l=2 modes, split by a large amount so that the m=+/-l modes overlap modes of adjacent l and n (in addition to a few radial modes). Such a pattern could be decoded if we had reliable frequency measurements for the observed modes. The current frequency identifications (by BFBCLS) are suggestive, but are sufficiently suspect that a detailed search for large splittings within their IDs is not advisable yet.

The Need for WET

While BFBCLS identify over 16 modes in their data, the data suffer from severe diurnal aliases that could easily have produced spurious mode identifications. BFBCLS do not specifically show a window function, and the Fourier transform plots are not of high enough resolution to show the diurnal aliases. Despite being reasonably high quality data from a large telescope, the fact that they observed from a single site at a low duty cycle (15 hours spread over 5 days) means that their diurnal aliases are severe. We should be able to do far better with multisite data.

Proposed observing plan

RA = 0h 16m 55.4s
Dec = +7:04:32
Epoch = J2000
B Magnitude 15.6

The principal goal of a campaign on PG0014 is to obtain good longitude coverage over a 2-week time period to adequately resolve the Fourier transform at sufficient frequency resolution for asteroseismic analysis. Most modes have amplitudes below 2 mmag, however - this is a low amplitude pulsator. With a B magnitude of 15.6, some large telescopes of the 2 meter class, distributed in longitude will be required, though smaller telescopes with CCD photometers can obtain useful data on this star. At an RA near 0h, and a near-equatorial declination of 7 degrees, the star is equally accessible from both Northern and Southern Hemisphere sites. The best time to observe this star surrounds dark time in September or October of 2004.

For the larger telescopes in our network, additional value can be obtained by obtaining multicolor time-series photometry. Even over the optical regions of the spectrum, such photometry may be able to constrain the order (l) of the mode. However, we stress that this would be an additional observational benefit but is not the focus of this WET proposal.


PG0014 has terrific potential for asteroseismic probing of the interior of a pulsating sdB star. It has many modes of pulsation, relatively short periods, and is ripe for further study to follow up on BFBCLS. Therefore, we propose to observe PG0014 for Xcov24 to obtain sufficient longitude coverage to resolve the pulsations. The resulting frequency list alone could be sufficient to verify the hypothesis that high l modes are present in PG0014 and/or test the prediction of large rotational splittings caused by a rapidly rotating core.


Brassard, P., Fontaine, G., Bergeron, P., Charpinet, S., Liebert, J., and Saffer, R. 2001, ApJ 563, 1013
Heber, U., Reid, I.N., & Werner, K. 1999, A&A 348, L25.
Kawaler, S. 1998, in 11th European Workshop on White Dwarfs, ed. J.-E. Solheim & E. Meistas, (San Francisco: ASP), p. 158
Kawaler, S., Sekii, T., & Gough, D. 1999, ApJ 516, 349.
Kawaler, S. & Hostler, S. 2003, in Keele Workshop on EHB Stars, ed. P. Maxsted
Reed, M.D. et al. (the WET collaboration) 2003, MNRAS, in press.
Sills, A. & Pinsonneault, M. 2000, ApJ 540, 489.

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