Jun 22nd: Finding Dipper Stars while looking for Exoplanets

By on June 22, 2019 in

Podcaster: Cosmic

Apogee Podcast

Title: The Apogee Podcast – Finding Dipper Stars while looking for Exoplanets

Links: astroandmusic.blogspot.com
Youtube: https://www.youtube.com/user/cosmiclettuce
Twitter: @AstroAndMusic
Email: cosmiclettuce@gmail.com

Description:   Cosmic chronicles the invention of the radial velocity spectrometer around 1868, and how that lead to the search for extrasolar planets in the 1990’s.  The Kepler mission changed the method of this search by using the transit method instead of the radial velocity method.  In examining the Kepler data, the Planet Hunters group found some unusual events associated with some stars, including the star KIC 8462852. also known as Boyajian’s or Tabby’s star.  Long-term dimming has been reported for this star, but the most recent measurements show this may not actually be happening.

Bio: Cosmic (aka Matt Cheselka) is an independent research astronomer and space musician.

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Hello!  This is Cosmic, and welcome to the Apogee podcast!  In these podcasts, I chronicle a single astronomical reference thread from the past to the present.  Many threads are possible — I’ve chosen just one.  These podcasts will take place at or near the date of the apogee which is when, along its orbit around the Earth, the Moon is furthest away.

The apogee for this podcast will take place tomorrow, 23 June 2019, at 7:52 UTC.  The lunar distance at that time will be 404548 km, which is 415 km further away than last apogee on 26 May, and 70 km further away than the next apogee on 21 July.  I also want to wish everyone a very happy Solstice!  This took place yesterday, 21 June, at 15:54 UTC.

If you have any suggestions for future podcasts, I would be happy to take a look at them.  I can be reached at cosmiclettuce AT gmail DOT com.

The music you hear in the background are my own compositions.  I hope you enjoy listening to space music as much as I do.

In 1967, R. F. Griffin reported on the creation of a photoelectric radial-velocity spectrometer.  These kinds of spectrometers allow astronomers to calculate the motions of astronomical objects by measuring very small wavelength shifts of spectral lines (like the hydrogen Balmer series) compared to laboratory measurements of those same lines.  In this paper, they comment that it had almost been 100 years since the first attempts to measure radial velocities were described by Sir William Huggins in 1868.  Although it was done by eye and was very crude, for one of the first times ever, Huggins examined and measured the spectra of the Orion Nebula (which he measured to be receeding at <10 miles per second from us), Sirius (receeding at 41.4 miles per  second), the Sun, and a comet.  In 1967, Griffin reports that their new electronic spectrometer had standard measurement errors of 0.2 km/s.

This fact is mentioned in a followup paper published in 1973 by nearly the same authors talking about measuring radial velocities to 0.01 km/s (10 m/s)!  One difficulty that this 1973 paper pointed out was that “instabilities and non-uniforities in the spectrograph components can lead to a relative displacement of the stellar and comparison spectra in the wavelength direction, which can result in relatively large systematic errors in radial velocities.”

This problem was picked up by Bruce Campbell et al in their 1988 paper describing a search for substellar companions of solar-type stars – using radial velocity measurements.  This group looked at twelve late-type dwarfs and four subgiants.  Two stars showed fairly significant variations in radial velocity, implying probable undiscovered stellar companions.  Seven of the remaining fourteen stars show small but significant long-term variations, implying possible small planetary bodies.  In order to make these kinds of very precise measurements, make sure your instrumentation is lined up before you

collect your data!

The work of Campbell et al was mentioned in 1995 by Walker et al who’d focused their attention on searching for Jupiter-mass companions to nearby stars using the radial velocity method.  In this paper, they report that despite having observed fourty-five nearby solar-type stars for many years, they’ve yet to find any Jupiter-mass or greater planets with short periods and circular orbits.

According to a breakthrough paper published in Nature in 1995, Mayor et al mention that Walker et al are just one of “several groups who’ve been examining the radial velocities of dozens of stars, in an attempt to identify orbital motions induced by the presence of heavy planetary companions”.  Mayor et al report, for the first time, the detection of a one-half Jupiter-mass companion orbiting the solar-type star 51 Pegasi at a distance of 0.05 astronomical units (about 7 million kilometers).

The information gathered about exoplanets from this work and others culminates in 2011 with the publication of The Exoplanet Orbit Database (EOD) by Wright et al.  At the time of the publication, the EOD contained orbital elements and other physical parameters for 427 exoplanets orbiting 363 stars.  Their website, exoplanets.org, was last updated June 2018 and contained information on 3263 exoplanets.

With the advent of the Kepler spacecraft, however, exoplanet discovery takes a major turn.  Many more exoplanets are discovered using the transit method. The exoplanet orbit database is mentioned in a 2012 paper by Fischer et al discussing a citizen science project called ‘Planet Hunters’.  This paper discusses the identification of two new planet candidates in the first month of Planet Hunters operations.

Because the volunteers for Planet Hunters look at every light curve by eye that is coming from the Kepler mission, serendipitous discoveries are inevitable.  Anomalous dips were seen in the star KIC (Kepler Input Catalog) 8462852.  This is what got Boyajian et al interested in 2016 when they took a

closer look at this object.  KIC 8462852 is one of the first “Dipper Stars”seen by Kepler.  After careful analysis, they conclude that while no planets or other large bodies are orbiting this star, it’s possible that exocomet or planetesimal fragments might be the cause of the deep aperiodic dips in the light curve.  This star is also known as ‘Boyajian’s Star’ or ‘Tabby’s Star’.

It is this very star that is one of the target objects in a very recent article (21 May 2019) published at the Research Notes of the Americal Astronomical Society (RNAAS) by Bradley E. Schaefer, discussing the fact that this star is a prototypical class 2 “Dipper Star”.  This class of dipper stars is a normal, isolated, main sequence star with no infrared excess. According to the article, Dipper Stars are “a recently discovered class of variable stars featuring a near-constant peak brightness with drops of 0.0003 [0.3 milli-magnitudes] to >4 mag, lasting from hours-to-months, with the dips being irregular in timing, shape, and/or depth.”  There were past reports that this star has faded by 20 percent in the past 100 years.  Measuring magnitudes accurately from photographic archives is very complex and subject to many errors.  This new paper to RNAAS is reporting new measurements using Harvard photographic plates that show that even though there have been many sudden dips in the brightness of this star, no long-term variablity has been seen over the past 100 years.

Thanks for listening!  Until next apogee, I bid you Peace.

End of podcast:

365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by Planetary Science Institute. Audio post-production by Richard Drumm. Bandwidth donated by libsyn.com and wizzard media. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at 365DaysOfAstronomy.org or email us at info@365DaysOfAstronomy.org. This year we will celebrates the Year of Everyday Astronomers as we embrace Amateur Astronomer contributions and the importance of citizen science. Join us and share your story. Until tomorrow! Goodbye!

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