Saturday, August 26, 2017

Likely Transiting Exocomets Detected by Kepler by Visual Lightcurve Inspection

      Image Credits:  Danielle Futselaar
     
KIC 3542116

https://arxiv.org/abs/1708.06069

In January 2017, amateur astronomer Tom Jacobs proposed the idea of visually surveying the complete Q1-Q17 Kepler lightcurve archive spanning 201,250 target stars for Data Release 25 (Thompson et al. 2016) with professional astronomers Saul Rappaport and Andrew Vanderburg along with amateur astronomer Daryll LaCourse. "The survey was conducted using the LcTools software system (Kipping et al. 2015), a publicly available Windows-based set of applications designed for processing lightcurves in a fast and efficient manner. Two primary components from the system were utilized; LcGenerator for building lightcurves in bulk and LcViewer for visually inspecting plots of the lightcurve for signals of interest." Tom Jacobs coordinated with citizen scientist Al Schmitt, author of LcTools, in generating the Kepler prime- field lightcurves in batches of ten thousand.

The team has worked on similar projects with the K2 campaigns. The survey lasted five months with one person (TJ) individually inspecting each lightcurve for interesting objects that were non periodic in nature. There were no predetermined search parameters. Everything was on the table and numerous flagged objects turned out to be data breaks and data processing anomalies (data glitches) upon subsequent analysis by Saul Rappaport and Andrew Vanderburg. Both veterans were indefatigable in supporting the search effort. On March 18th, KIC 35421116 was initially flagged for its three aperiodic transits and subsequently labeled and analyzed as exocomet candidates.



Initially, there was a lot of back and forth discussions on just what the three events were. Concern about data processing anomalies, stellar variability, star dimming (photometric variability), cosmic ray hits (SPSD) and contamination by the neighboring star KIC 3542117 had to be addressed. These concerns were examined in detail and later ruled out as more astronomers, with the specialized expertise, joined the team. Saul Rappaport independently narrowed the possibilities of event type to exocomets due to the asymmetric shapes, shallow depths and lack of clear periodicity. Jon Jenkins successfully advocated for three smaller events in the lightcurve that were eventually considered by the team to be exocomet transits with a possible stellar variability caveat due to their shallow depths and low signal to noise ratio.

Knowing that exocomet-like targets exist in the Kepler data, a second exocomet candidate was flagged in the lightcurve of KIC 11084727 on May 1st, though similar lightcurve profiles would not have been missed during the survey. Attaching an exocomet label to the event proved to be very exciting.



Without the visual survey it is highly unlikely that these two stars would have been flagged even though exhaustive algorithm machine surveys have scoured through the Kepler prime-field data looking for transits over the years. Viewing comet transits in the Kepler data was not thought possible until now and they appear to be rare events likely due to instrument sensitivity and orbit profile viewed by Kepler:

 "With regard to the number of comets that should be seen crossing the disk of the host star, this rate should depend heavily on whether the infalling comet orbits are distributed roughly isotropically (lower rate) or if the reservoir of bodies producing the dusty tails has orbits that are coplanar with the angular momentum vector of the system (higher rate if the observer lies in this plane)."

 "Present technology does not enable us to view images of these kilometer-size infalling bodies" SETI Institute talk, "Exocomets: Now you see them, now you don't" by Dr. Barry Welsh January 23, 2017.

To date: "Radio observations, in particular detections of circumstellar CO emission around stars such as HD 181327 (Marino et al. 2016), Eta Corvi (Marino et al. 2017), and Fomalhaut (Matra et al. 2017) have been attributed to the presence of substantial populations of minor bodies at large orbital separations. Another sensitive method for detecting and understanding populations of extrasolar minor planets is through time-series spectroscopy, rather than time-series photometry (e.g., as performed by Kepler)."

Exocomet candidates are then said to have been observed indirectly:"Growing evidence for the existence of large populations of extrasolar minor bodies orbiting other stars has come from the detection of anomalous absorption features in the spectra of at least 16 A-type stars where Falling Evaporating Bodies (`FEBs') are proposed to randomly cross the observing line of sight (e.g., Ferlet et al. 1987; Beust et al. 1990; Welsh & Montgomery 2015)."

According to Barry Welsh, "a spectrographic study would clinch their case (Kepler) for comets or show they found rocks and dust" (Wonky Signals from distant stars could be signs of exocomets by Jesse Emspak). What we are now seeing is a possible synergistic blending of observation modalities for validating comets outside our solar system. The upcoming TESS mission may provide more opportunities to observe exocomets and expand our knowledge of these early pristine objects of solar system evolution.

A paper written by a French team in 1999, "Photometric Stellar Variation due to Extra-solar Comets" predicted the detection of exocomets by their dusty tails ( A. Lecavelier des Etangs, A. Vidal-Madjar, and R. Ferlet 1999).

"We performed detailed numerical simulations of stars’ occultations by extra-solar comets. We extracted the apparent photometric variations of the central stars due to these putative comets. We have shown that: 1) Extra-solar comets can be detected through photometric variations due to occultation by dusty tails. In many cases, the light curve shows a very particular “rounded triangular” shape. However, in some remaining cases, the curve can mimic a planetary occultation. 2) The photometric variations due to cometary occultations are mainly achromatic. This property will allow to discriminate the occultations by comets from intrinsic stellar variations. However, the confusion with planetary occultations cannot be efficiently removed by color measurements in the optical. 3) The number of detections which can be expected from a large photometric survey of several tens of thousand of stars at high accuracy (10−4 ) is of the order of several hundreds of occultations per year."

In 2017 we may be seeing their predictions coming true.


















































Wednesday, January 11, 2017

Resurvey of Kepler Legacy Stars Long and Short Cadence Data

          KIC 1028018 LcTools Lightcurve Viewer

Citizen Scientists now have a second chance to survey the Legacy Kepler light curves from their computer den using the free LcTools program by Allan R Schmitt. The light curve resolution is outstanding. This year, using the LcTools generator, the Kepler Long Cadence light curves will be made available as text files in batches of ten thousand stars. Visit Allan R Schmitt's website for the latest release.




An Automated Search for Transiting Exocomets- HD 182952 (KIC 8027456)

A team of astronomers led by Grant M. Kennedy , discovered a potential third comet system in the Kepler prime field data of HD 182952 (KIC...