least 2003" – Two stars since the 1970s" • Monitor pulse arrival times for about a dozen DAVs (hydrogen- atmosphere WDs)" – Similar to the pulsar timing method" • > 95% of all stars in our Galaxy (including our Sun) will be DAVs" • Pulsation periods 100-500 s" – We expect the period change in these pulsations to be very slow (< 1 μs yr-1)" – After 9 years we are sensitive to Jupiter-mass planets from 2-10 AU" Mullally et al. 2008, ApJ 676 573 JJ Hermes, UT-Austin, 221st AAS GD 244, typical DAV in our sample"
Some DAVs have been observed for 35+ years, including G117-B15A" • The 215.2 s mode in that star produces an extremely stable rate of change of period with time" • This dP/dt is in line with expectations of cooling for this ~12,000 K WD" • The inﬂuence of a Jupiter-mass planet at 5 AU would cause an unmistakable 10 s peak-to-peak modulation in the (O-C) diagram" JJ Hermes, UT-Austin, 221st AAS S.O. Kepler 2012, private communication
the secular trend from cooling and look for periodic modulation" • We are nearly able to exclude a Uranus-mass planet at Uranusʼs distance" JJ Hermes, UT-Austin, 221st AAS Kepler et al. 2005, ApJ 634 1311 Window" Residual Periodogram" (O-C) Residuals"
AAS • DA WD evolution should be simple, dictated by cooling" • Expected rate of dP/dt < 10-14 s s-1 ( < 0.3 μs yr-1 ) for all radial order (k) and spherical degree (l) (Bradley et al. 1992, ApJ 391 L33)" • The high-amplitude mode in G117-B15A (and R548) behaves this way" • However, the WDs have some surprises in store for us"
AAS Hermes et al. 2013, ApJ submitted • The assumption that all modes in all DAVs are extremely stable (and that dP/dt < 10-15 s s-1) is not universal! • There is a physical effect operating at a non-cooling timescale in this DAV" • As with G117-B15A, we can remove this secular evolution to search for planetary companions by looking for periodicity in the four modes present" • Again, we can exclude Jupiter-mass planets over a wide range of possible orbits (at least 3-10 AU)" Window" Residual Periodogram"
221st AAS tant (Benvenuto et al. 2004), as well as provide useful on the mass of the hypothesized axion or other super- particles (Isern et al. 1992; Co ´rsicoet al. 2001; Bischoff- 2007). et is in orbit around a star, the star’s distance from the ange periodically as it orbits the center of mass of the ystem. If the star is a stable pulsator like a hDAV, this a periodic change in the observed arrival time of the stable pulsations compared to that expected based on planet mass, MÃ is the mass of the WD, c is the speed of light, and i is the inclination of the orbit to the line of sight. In common with astrometric methods, the sensitivity increases with the orbital separation, making long-period planets easier to detect given data sets with sufﬁciently long baselines. In 2003 we commenced a pilot survey of a small number of DAVs in the hope of detecting the signal of a companion planet. We present here a progress report of the ﬁrst 3Y4 yr of observa- tions on 12 objects, as well as presenting limits around three more objects based partly on archival data stretching as far back as 1970. For one object we ﬁnd a signal consistent with a planetary Sample FT of GD 66 from a single 6 hr run. The larger amplitude eled with their periods. The peaks at 271 and 198 s are composed of sely spaced modes separated by approximately 6.4 Hz that are not is FT. Fig. 2.—The OÀC diagram of the 302 s mode of GD 66. The solid line is a sinusoidal ﬁt to the data. f2 Mullally et al. 2008, ApJ 676 573 • The 302.77 s mode showed evidence for periodic behavior, and a 2MJ planet in a 4.5-year orbit was proposed by Mullally et. al 2008" • Nearly ﬁve years later, how is “GD 66b” looking?"
221st AAS Hermes et al. 2013, in prep. • As “expected” the (O-C) diagram for f2 has turned over, and there is clearly a periodic modulation to the arrival times of this pulsation mode" – The period has since been reﬁned slightly (deﬁned by the best-ﬁt linear trend)" • This modulation is currently consistent with a 1.1 MJ sin i planet at 2.2 AU (4.1 yr); there is no amplitude modulation, especially on this timescale"
221st AAS Hermes et al. 2013, in prep. • We have been able to construct an (O-C) diagram for the highest-amplitude mode at 271.71 s, which is the m=0 component of a detected triplet (we simultaneously ﬁt all 3 components, using several nights of data)" • This mode also shows a 4.0-year modulation consistent with a 1.2 MJ sin i planet!"
221st AAS Hermes et al. 2013, in prep. • Complication: The best-ﬁt sine curves to f1 and f2 are nearly π out of phase" • An external companion would modulate all modes identically! • While discouraging for the planetary hypothesis, this is likely telling us something very interesting about the physics of pulsations in this star."
221st AAS Dalessio et al. 2013, ApJ in press – 14 – 1995 2000 2005 2010 2015 −250 −200 −150 −100 −50 0 50 100 150 200 250 Time (Years) O−C (s) Fig. 2.— O − C of pulsation frequency D. The dashed lines are located at 0, P, and −P. The blue line is the best model ﬁt for Π = 12.9yrs. The red lines indicate the boundaries of the one sigma likelihood prediction of the model not including the 1994 data. Note that a ﬁrst order polynomial has been removed from the data. – 15 – 1995 2000 2005 2010 2015 −300 −200 −100 0 100 200 300 Time (Years) O−C (s) Fig. 3.— O − C of pulsation frequency E. The dashed lines are located at 0, P, and −P. The blue line is the best model ﬁt for Π = 12.9yrs. The red lines indicate the boundaries of the one sigma likelihood prediction of the model not including the 1994 data. Note that a ﬁrst order polynomial has been removed from the data. • GD66 is not the only pulsating white dwarf that shows such periodic behavior:" – James Dalessio (U. Delaware) has observed a similar effect in a DBV (He-atmosphere), EC 20058-5234" • Both the 204.6 s (top) and the 256.9 s (bottom) modes have underlying 12.9-year periodic changes" – Just as with GD 66, the changes are π out of phase with each other!" • Again, this cannot be an external effect; it is rather evidence for some non-cooling timescale in pulsating WDs" 204.6 s mode" 256.9 s mode"
Planet JJ Hermes, UT-Austin, 221st AAS Periodic modulation in the pulse arrival times of GD 66 had been interpreted as evidence of a possible planetary-mass companion." " Continued observations show that two modes in the star are in fact modulated at a 4-year period." " Unfortunately both modes are nearly exactly out of phase with each other. This cannot be space motion caused by a planetary companion." " Pulsating white dwarf stars can still be used to search for planets, but they are not as well behaved as we wanted them to be!"