An HST search for planets in the lower main sequence of the globular cluster NGC 6397

Abstract

Searches for planetary transits carried out in open and globular clusters have to date yielded only a handful of weak, unconfirmed candidates. These results have been interpreted as either being insignificant, or evidence that the cluster chemical or dynamical environment inhibits planetary formation or survival. Most campaigns have been limited by small sample statistics or systematics from ground-based photometry. We performed a search for transiting planets and variables in a deep stellar field of NGC 6397 imaged by HST-ACS over 126 orbits. We analyzed 5078 light curves, including a careful selection of 2215 cluster-member M0-M9 dwarfs. The light curves were corrected for systematic trends and inspected using several tools. No high-significance planetary candidate is detected. We compared this null detection with the most recent results from Kepler, showing that no conclusive evidence of lower planet incidence can be drawn. However, a very small photometric jitter is measured for early-M cluster members ( 2 mmag on 98% of them), which may be worth targeting in the near future with more optimized campaigns. Twelve variable stars are reported for the first time.

The M 4 Core Project with HST: I. Overview and first epoch

“…Figure 12 demonstrates that for the brightest stars we are able to achieve a photometric accuracy of ∼ 5 millimagnitudes, i.e., still significantly worse than shot-noise (∼ 3 millimag). Using techniques similar to those presented in Nascimbeni et al (2012), we expect our final photometry to further approach the theoretical limit.…”

“…M 4 is also dynamically old and we expect most binaries to have sunk into its central region. By rescaling the yield of EBs estimated by Nascimbeni et al (2012) in NGC 6397, we expect a few tens of detections from our data set. Some of them will be SB2 detached systems bright enough to be suitable for a ground-based RV follow-up on 8-10 m class telescopes.…”

“…Some of us previously performed a search for transiting planets and variables in a deep stellar field of NGC 6397 imaged by HST-ACS/WFC over 126 orbits (Nascimbeni et al 2012). The light curves were corrected for systematic trends and inspected with several tools, including a boxfitting least-square algorithm (BLS, Kovács et al 2002) to perform transit-finding in the BLS periodogram.…”

“…The observing strategy is still far from optimal for transit searches, since this program is designed for other purposes. Nevertheless, the advantages of GO-12911 data over those used by Nascimbeni et al (2012) are evident (faster cadence, larger temporal coverage, larger number of imaged cluster members). We will achieve a ∼ 40 % efficiency in detecting transiting "hot Jupiter" planets with orbital periods shorter than 4 days.…”

Did a stellar fly-by shape the planetary system around Pr 0211 in the cluster M44?

“…Although several planets have been detected, it is still far from clear whether planets are as common around stars in clusters as around field stars or not. Out of the ∼3000 exoplanets detected so far, only fourteen have been discovered in open clusters and the number of detected planets varies considerably from cluster to cluster: For example, three planets were detected in Hyades (Sato et al 2007;Lovis & Mayor 2007;Quinn et al 2014), five in M 44 -Praeceps (Quinn et al 2012;Malavolta et al 2016;Obermeier et al 2016), and four in M 67 (Brucalassi et al 2017), whereas, for example, no planets were detected so far in 47 Tucanae (Gilliland et al 2000;Weldrake et al 2005), NGC 2301 (Howell et al 2005) and NGC 7789 (Bramich & Horne 2006) and other investigated clusters (Nascimbeni et al 2012). Therefore these results are interpreted by the respective authors from "planets are very rare" in clusters to "they are as common or even more than around field stars".…”