Laura Malis
Under the Supervision of Dr. Aaron Geller
Summer Project CIERA Summary
Northwestern University

Despite our knowledge of a single star evolutionary sequence, we are becoming increasingly aware of stars that depart from this sequence. While observing stars within star clusters, astronomers found stars that do not follow the standard stellar evolution pathway, and have dubbed some of these stars as sub-subgiants. Recently, the NASA Kepler Satellite observed a large sample of stars in the Galactic field (outside of star clusters); some of these stars appear to have similar surface temperatures and gravities as the sub-subgiants that were previously identified in star clusters. Particularly some and possibly all of the Kepler "No-Man’s-Land" (Huber et al. 2014) stars may truly be sub-subgiants. Some authors suggest that the surface temperatures and gravities derived for these stars from observations are inaccurate, and they cast these stars off as mistakes within data. However, our further analysis shows that the "No-Man's-Land" stars also share other important characteristics with the sub-subgiants, including a high frequency of photometric variables that tend to have short periods. The "No-Man’s-Land" stars are also filled with fascinating objects such as eclipsing binary stars and Kepler Objects of Interest (KOI, i.e., exoplanet candidates). We conclude that many, and perhaps all, of the Kepler "No-Man’s-Land" stars may be true sub-subgiants, the first of which to be discovered outside of star clusters.

What was the Kepler Mission?

The Kepler Mission was an earth trailing, heliocentric space observatory launched in March of 2007. Kepler's purpose was to survey our region of the Milky Way galaxy in the search of other Earth-size planets that could possibly be habitable. Ultimately, Kepler found 2327 confirmed planets. For more information on the Kepler Mission, click here.

The Kepler space observatory was launched on March 7, 2009.
Ultimately, we compared 152,644 celestial objects using Python and Jupyter Notebooks. Starting with a list of the "No-Man's-Land" stars from Huber et al., we wanted to see if stars within this field coincided with any stars previously searched for photometric variability. Specifically we used the results from McQuillan et al. (2014), who provide light curve analyses and rotation periods for a large fraction of the Kepler sample. Using Python notebooks, we cross matched the "No-Man's-Land" (7,909) stars with stars in McQuillan's Table 1 (34,030 with rotation period measurements) and then once more in Table 2 (99,000 without reliable rotation periods) . Our ultimate result was a table that consisted of all the matched stars (3,560) across the 3 sets of data. With these data, we then created histograms and charts to further analyze the characteristics of these stars using modules such as MatPlotLib and Numpy.

McQuillan et al. only included (presumed) main-sequence stars in their sample, and excluded known eclipsing binaries (EB) and KOIs. We therefore also matched our "No-Man's Land" sample to the Kepler EB and KOI tables. The cumulative KOI contains current results of different searches of the Kepler light curves, and the list of archived eclipsing binaries contains all known eclipsing binaries in the Kepler primary survey. Using the same method of matching as done with the McQuillan tables and the "No-Man's-Land", we compiled a table that consisted of all the matches of the three original tables, a match between these three tables and the Kepler identification numbers included in the EB table, and whether or not the star was a confirmed or candidate KOI. (For the purposes of our research, all false-positive results have been removed.)

Of the 7,909 "No-Man's Land" stars, 5,803 are matched to either the McQuillan paper, the EB table or the KOI table, leaving 2,106 stars that have not been investigated for photometric variability. In the future, we may plan to look further into these stars by plotting light curves.


Numbers and Figures

Teff vs logg for the "No-Man's Land" stars (blue points). All stars in the "No-Man's Land" are located to the red (right) of the reddest possible isochrone for the Galactic field. Yellow points show stars that are in both "No-Man's-Land" and McQuillan's Table 1. Green points show stars that in both "No-Man's Land" and McQuillan's Table 2. We can see that a portion of the "No-Man’s-Land" stars have yet to be further analyzed and are missing periods of rotation. We took these IDs that didn't match the McQuillan tables and cross referenced them against KOIs and eclipsing binaries. What we found was that many of these stars had matches in KOI and eclipsing binary databases.
Cumulative period distribution of all stars with rotation periods from McQuillan et al. (Blue) compared to only the stars with rotation periods in "No-Man’s Land" (Green). Based on the Cumulative Distribution of periods and K-S test, we can conclude that these two distributions were drawn from different parent populations. What we can infer from this is that the stars that match with "No-Man’s-Land" are considerably different from the rest of the sample, and could possibly be sub-subgiants.
Cumulative period distribution of all stars with rotation periods from McQuillan et al. (Blue) compared to only the stars with rotation periods in "No-Man’s Land" (Green). Same data points as above graphed as a histogram.

What we can conclude from the stars from McQuillan et. al (2014) is that some, if not all, of these stars are sub sub-giants. Our cumulative distribution of period rotations leads us to conclude that these stars are drawn from different parent sources. We can conclude from the above graphs and charts that there are places of obvious overlap that have not been sufficiently researched, and even possibly overlooked. Furthermore, these stars are not only possible sub-subgiants, but some of these identification numbers found McQuillan et. al (2014) were also evident in a database of eclipsing binaries and KOI's (Kepler Objects of Interest). This could, with further research, allow us to use the already charted data about these already identified stars to help further our knowledge about stars with missing data, such as periods of rotation.

As we continue to find intersections between "No-Man's-Land" and other types stars and planets, we would like to look further into certain KOI's and eclipsing binaries. Some of our stars host anywhere from 2-5 planets. To further study these these "No-Man's-Land" stars with planets, especially where we find multiple overlaps in our databases, we are hoping to soon investigate light curves and the additional available characteristics about these stars with planets. We also plan to analyze the light curves for the sample of "No-Man's Land" stars that are not in McQuillan's sample, so that we can obtain a complete sample of rotation periods for the "No-Man's Land" stars.

I am in the class of 2017 this school year at Adlai E. Stevenson Highschool in Lincolnshire, Illinois. I hope to continue research in astronomy and computer science, and maybe pursue these fields as I continue on to higher education. If you have any questions about my research or would like to dicuss any concerns, feel free to reach me by email at lmalis7 [at] students.d125.org.