Many areas of stellar astronomy are of interest to our group, such as pulsations of variable stars (asteroseismology), stellar populations and star formation histories of galaxies, cataclysmic variable studies, and even gamma-ray bursts.
Asteroseismology
Asteroseismology is the study of pulsations on other stars, using many of the techniques developed for solar oscillations. We want to learn about properties of stars, such as internal rotation, that cannot be found by any other means. The principle difference with the Sun is that we cannot spatially resolve most stars because they are so distant, and thus they are point sources of light. This complicates the analysis of the pulsation spectrum because far fewer frequencies are detectable. This is still a very young and promising field of stellar astronomy, with many interesting problems to study.
The asteroseismic community, over the next few years, will be provided with unprecedented space-based data. Satellite instruments like MOST and CoRoT have already been operating for several years, and the recently launched Kepler mission promises outstanding data. A less expensive but very flexible ground-based observing project, SONG, will likely become operational in the next 3-5 years as well.
At NMSU we have both photometric and spectroscopic observing opportunities at Apache Point Observatory. We are involved in Kepler projects for data analysis and follow-up ground observations of various target stars.
We have ongoing collaborations with Joyce Guzik (LANL), Markus Roth (Kiepenheuer Institute), J. Christensen Dalsgaard (Aarhus University, Denmark), Travis Metcalfe (p). We propose to conduct a study of the internal properties of fifty-five stars located near the top of the red giant branch. All of these stars are currently on the Kepler drop list so they will have to be added to the cycle 2 observing list. This study is complementary to the work of Bedding et. al. (2010), that analyzed the pulsation spectra of the low-luminosity red giant stars. Our program stars have effective temperatures and surface gravities of less than 3600K and log(g) = 1.0, respectively. Project goals are 1) to quantify the range of pulsation spectra found in upper red giant branch stars, 2) to use state-of-the art FAMIAS software to determine the values of several key global and interior properties, and 3) to determine the decay rate (if any) of the observed pulsation modes. Parameters to be measured include masses, ages, metal contents, convective overshoot parameters, hydrogen contents, radii, surface rotations, and rotation profile. It has long been speculated that oscillations, similar to the p-modes in the Sun, are stochastically excited by convective turbulence in red giant stars. Observational studies support this assertion, however, the measured decay times vary by a significant amount. Estimates lie in the range of days to weeks, but an even longer time frame might be possible. Since little is known about the long term stability of the oscillations in these stars, and because of the necessity of removing the longer term pulsations from our light curves, a full year of data is requested.
The 55 program stars fall into four classes, 1) stars whose light curves are representative of the nearly 500 stars in the Kepler drop list that are located on the upper red giant branch, 2) stars whose light curves are more complex, 3) stars which possess a rapidly varying, but periodic signal, and 4) stars that possess small dips or peaks in their light curves that are superimposed on a longer term variation. The first of these groups is intended to provide reference values against which the other derived parameters can be compared. The second group was chosen from a visual inspection of the light curves of all 500 drop list stars. Their complex light variations suggest the presence of several frequencies. The third group likely consists of giant star binaries. The last group consists of stars that possess a short term periodic decrease or increase in the star's brightness. The approximate period of their dips/rises is about 120 ksec. We believe this phenomenon has not previously been noted. Stars possessing this feature appear to be confined to a small region of the HR diagram, suggesting it has a physical origin.
The evolution of the massive B stars is driven by events that take place deep within their interiors. Although the physical processes operating there are thought to be well known, small uncertainties in model inputs involving the opacities, the amount of mixing, and the interior rotation profile, have a significant impact on a star's evolution. Scaled solar values are often used for the first two of these quantities. However, their applicability to intermediate mass stars is questionable. Similarly, although main sequence B stars are rapid rotators, rotation is either not included in evolutionary codes or it is only treated in a simplified manner. To accurately model the structure and evolution of these stars, their metal contents, convective overshoot parameters, and rotation profiles must be determined. By using a star's pulsation spectrum, the tools of asteroseismology permit this to be done. Ground-based programs have attempted to measure these quantities, but they generally detect too small a number of pulsation frequencies. The pioneering space-based WIRE (Wide-Field InfraRed Explorer) and MOST (Microvariability and Oscillating Stars) satellites have been considerably more successful and have detected dozens of frequencies. However, neither of these micro-satellites can observe a significant sample of B stars because they are limited to very bright stars (M<6).
The primary goal of this effort is to determine the pulsation spectra of up to 122 relatively bright B stars in the Kepler database to a limiting amplitude of 0.02 mmag. A secondary goal is to demonstrate the value of these Kepler pulsation spectra by obtaining ground-based multi-color light curves for up to three large amplitude stars in this sample. This combined dataset will allow the modes of the larger amplitude pulsations to be determined using their multi-color amplitude ratios and will permit the masses, ages, metal content, convective overshoot parameter, hydrogen content, radii, surface rotation, and rotation profile of these stars to be obtained.
Eclipsing binaries currently serve as the most accurate method of obtaining masses and
radii of stars, with resulting errors usually less than 1%. At the beginning of of 2007, the
lower main-sequence suffered from a lack of known low-mass, main-sequence, (M < MSun,
Teff < 5500 K), eclipsing binary stars, with only nine well-studied systems. As outlined by
L'opez-Morales (2007), (see Table 2 and references therein), those systems revealed that the
observed stellar radii are consistently ~10% larger than predicted by stellar models (Baraffe
1998) for 0.3 MSun < M < 0.8 MSun. This discrepancy either reveals a flaw in the stellar models,
or is possibly a result of varying metallicity and/or enhanced magnetic activity due to the
binary nature of the observed systems (Morales et al. 2008). In the latter case, shorter-period
binary systems, with the stellar rotation rate enhanced by the revolution of the system, would
be expected to show greater activity and thus larger radii than longer-period systems.
Unfortunately, although a few more systems have been found and well-studied since
2007, (Coughlin & Shaw 2007; Becker et al. 2008; Blake et al. 2008; Devor et al. 2008;
Shkolnik et al. 2008), of all the studied systems to date only 8 systems have have 1.0 < P
< 3.0 days, and only one has a larger period (P = 8.4 days). This is mostly due to the
fact that current photometric surveys such as NSVS, TrES, and OGLE, are either cadence,
precision, magnitude, or number limited, and thus not sensitive to long periods. For systems
with component masses of 0.5 MSun, they are expected to synchronize in less than 0.1 Gyr
for periods less than 4 days, and in less than 1 Gyr for periods less than 8 days (Zahn 1977,
1994). Thus, the discovery of long-period systems of P > 10 days is crucial to studying the
effects of binary period and thus stellar activity on the mass-radius relation of the lower
main-sequence. Furthermore, the discovery of more low-mass eclipsing systems, even with P
< 10 days, would be extremely helpful in further defining the empirical mass-radius relation
and isolating the effects of metallicity across the entire range of component masses.
Of ultimate application, the precise calibration of the mass-radius relation for the lower-
main sequence is critical for deriving precise extrasolar planet radii from transits across these
stars, as one assumes the radius of the host star in ï¬tting a transit curve. As K and M dwarfs
make up the ma jority of stars in the galaxy, and thus in the Kepler field of view, and since
transits across these late types are more sensitive to lower mass planets, it is of even further
importance to discover and study these systems.
The delta Scuti and gamma Doradus variables are main sequence (core hydrogenburning)
stars with masses somewhat larger than the sun (1.2 to 2.5 solar masses). The
gamma Dor stars, having cooler effective temperatures, are pulsating in nonradial gravity
modes with periods of near one day, whereas the delta Sct stars are radial and nonradial
p-mode pulsators with periods of order one hour. Because of the near one-day periods of
gamma Dor stars, satellite observations over a few weeks are preferred to ground-based
single sites or networks to find multiple periods in these stars.
Theoretically, gamma Dor and delta Sct pulsations would not be expected to co-exist.
The gamma Dor g-mode pulsations are explained by a convective blocking mechanism
that produces pulsation driving at the base of an envelope convection zone extending to
temperatures of several hundred thousand Kelvin. The delta Sct p-mode pulsations are
explained by driving in the helium ionization region in the envelope at about 50,000 K by
increased opacity in this layer regulating radiation diffusion (the kappa effect). The kappa
effect should not operate in the convection zones of gamma Dor stars, as convection
instead of radiation is efficiently transporting the star's energy outward.
Nevertheless, about half of the gamma Dor stars lie just within the theoretical delta Sct
instability strip, and a few hybrid gamma Dor and delta Sct pulsators have been reported. The pulsations of one previously reported hybrid
HD 209295 (Handler and Shobbrook MNRAS 333, 2002) could be explained by tidal
interactions of a binary companion, while another hybrid candiate HD 8801 (Henry and
Fekel, AJ 129, 2005) is a single Am star with abundance peculiarities.
We will observe several stars accessible by Kepler with effective temperatures and
abundances near the boundary of these two variable star types that are promising
candidates for hybrid pulsators. With a year (or even a quarter year) of photometric
monitoring, a number of p- and g-mode frequencies that are ubiquitously predicted in
main-sequence A-F stars could be determined. With enough modes, we could use
asteroseismology to provide constraints on the internal structure of these stars, and learn
more about how these two types of pulsations could coexist. Alternatively, verifying that
the target stars are not hybrid is also useful if a number of frequencies can be found to
constrain the structural conditions that do not produce this behavior. It is likely that
asteroseismology of such stars will lead us to a better understanding of the physics of
time-dependent convection, opacities, helium and element diffusive settling, and
rotational mixing.
Binary Stars
Red Giant stars in eclipsing binary systems are exciting candidates for testing stellar evolution models. We are involved in an international project to exploit these astrophysical objects.
Other Kepler Asteroseismic Studies
Characterizing the top of the Red Giant Branch using Kepler drop-list stars
Measuring the sub-millimagnitude Frequency Spectra of Pulsating B Stars
Measuring the Masses and Radii of the Lower Main Sequence: Identification of New Eclipsing M Dwarfs
A Search for Hybrid Gamma Dor/Delta Scuti Pulstating Variable Stars (lead: Joyce Guzik, LANL)
