Many photometric time-domain surveys are driven by specific goals, such as searches for supernovae or transiting exoplanets, which set the cadence with which fields are re-imaged. In the case of the Palomar Transient Factory (PTF), several sub-survey
s are conducted in parallel, leading to non-uniform sampling over its $sim$$20,000 mathrm{deg}^2$ footprint. While the median $7.26 mathrm{deg}^2$ PTF field has been imaged $sim$40 times in textit{R}-band, $sim$$2300 mathrm{deg}^2$ have been observed $>$100 times. We use PTF data to study the trade-off between searching for microlensing events in a survey whose footprint is much larger than that of typical microlensing searches, but with far-from-optimal time sampling. To examine the probability that microlensing events can be recovered in these data, we test statistics used on uniformly sampled data to identify variables and transients. We find that the von Neumann ratio performs best for identifying simulated microlensing events in our data. We develop a selection method using this statistic and apply it to data from fields with $>$10 $R$-band observations, $1.1times10^9$ light curves, uncovering three candidate microlensing events. We lack simultaneous, multi-color photometry to confirm these as microlensing events. However, their number is consistent with predictions for the event rate in the PTF footprint over the surveys three years of operations, as estimated from near-field microlensing models. This work can help constrain all-sky event rate predictions and tests microlensing signal recovery in large data sets, which will be useful to future time-domain surveys, such as that planned with the Large Synoptic Survey Telescope.
Over the past 40 years, observational surveys have established the existence of a tight relationship between a stars age, rotation period, and magnetic activity. This age-rotation-activity relation documents the interplay between a stars magnetic dyn
amo and angular momentum evolution, and provides a valuable age estimator for isolated field stars. While the age-rotation-activity relation has been studied extensively in clusters younger than 500 Myr, empirically measured rotation periods are scarce for older ages. Using the Palomar Transient Factory (PTF), we have begun a survey of stellar rotation to map out the late-stage evolution of the age-rotation-activity relation: the Columbia/Cornell/Caltech PTF (CCCP) survey of open clusters. The first CCCP target is the nearby ~600 Myr Hyades-analog Praesepe, where PTF has produced light curves spanning more than 3 months and containing >150 measurements for ~650 cluster members. Analyzing these light curves, we have measured rotation periods for 40 K & M cluster members, filling the gap between the periods previously reported for solar-type Hyads (Radick et al. 1987, Prosser et al. 1995) and for a handful of low-mass Praesepe members (Scholz et al. 2007). Our measurements indicate that Praesepes period-color relation undergoes at transition at a characteristic spectral type of ~M1 --- from a well-defined singular relation at higher mass, to a more scattered distribution of both fast and slow-rotators at lower masses. The location of this transition is broadly consistent with expectations based on observations of younger clusters and the assumption that stellar-spin down is the dominant mechanism influencing angular momentum evolution at ~600 Myr. In addition to presenting the results of our photometric monitoring of Praesepe, we summarize the status and future of the CCCP survey.
We have conducted a search for pulsar companions to 15 low-mass white dwarfs (LMWDs; M < 0.4 M_Sun) at 820 MHz with the NRAO Green Bank Telescope (GBT). These LMWDs were spectroscopically identified in the Sloan Digital Sky Survey (SDSS), and do not
show the photometric excess or spectroscopic signature associated with a companion in their discovery data. However, LMWDs are believed to evolve in binary systems and to have either a more massive WD or a neutron star as a companion. Indeed, evolutionary models of low-mass X-ray binaries, the precursors of millisecond pulsars (MSPs), produce significant numbers of LMWDs (e.g., Benvenuto & De Vito 2005), suggesting that the SDSS LMWDs may have neutron star companions. No convincing pulsar signal is detected in our data. This is consistent with the findings of van Leeuwen et al. (2007), who conducted a GBT search for radio pulsations at 340 MHz from unseen companions to eight SDSS WDs (five are still considered LMWDs; the three others are now classified as ordinary WDs). We discuss the constraints our non-detections place on the probability P_MSP that the companion to a given LMWD is a radio pulsar in the context of the luminosity and acceleration limits of our search; we find that P_MSP < 10 +4 -2 %.
