The reasons for changes in the brightness of a star can be divided into two categories: (1) orbiting companions or (2) stellar astrophysics.
(1) In principle, the variability from orbiting companions (this includes eclipsing binaries or transiting planets) should be as regular as clockwork. In practice, the variability can deviate from clockwork regularity if stellar binaries get too close together, if there are multiple transiting planets, if there is additional background electronic noise or astrophysical noise.
(2) Brightness variations caused by physical processes internal to the star (stellar astrophysics) can arise from pulsations of the star, starspots or flares. Flares are random spikes in the light curve brightness. Pulsations from stars (like RR Lyraes) are quasi-periodic: they can appear to be regular for a while and the cycles are relatively short (generally hours to a day or so). The Figure below shows two variable stars with short periods that might be best classified as “variable” and “pulsating.” These could be short period binary systems – this could quickly be verified with follow-up observations.
Starspots produce complex variations. As the star spins, the spots rotate in and out of view with a periodicity of a day or two (for the most rapidly spinning stars) to several days for slowly rotating stars (the Sun has a rotation period of 25 days). Starspots can form at different latitudes on the star. Since some latitudes rotate faster, spots can show multi-cyclical variations. The light curves below might be best classified as variable and irregular. However, a case could be made for classifying the light curve in the figure below (and left) as variable and regular. Even though the amplitude of the curves changes, the time from one peak to the next is about the same.
Like many of you, I am extremely eager to find those tiny, elusive dips in the TESS lightcurves that reveal the existence of a distant, undiscovered, alien world. However, even though planets are my main focus, they are not the only interesting objects that we are able to find using TESS.
I have recently been talking to astronomers at the KU Leuven, in Belgium, who use the TESS lightcurves to study stars, their spots and pulsations as well as the architecture and behaviour of multiple star systems.
The lighcurves of these systems often boast beautiful patterns! Cole Johnston from the KU Leuven explains some of the systems behind these lightcurves:
Gamma Doradus variables are stars slightly more massive than the Sun with temperatures between 6,500K and 7,500K (the Sun is around 5,777K). You can recognise their lightcurves due their characteristic fluctuations in brightness that last from a few hours to several days. The fluctuations are caused by pulsations known as gravity-mode pulsations, which are waves that behave in the same way as surface waves in the ocean. These waves affect the surface temperature of the stars, changing the brightness that we observe.
Delta Scuti (also known as dwarf Cepheid variables) stars are hot, young (A-F type) stars that are ~1.5 to ~2.5 times the mass of our Sun. These stars can have a single strong pulsation or potentially hundreds of smaller pulsations with periods ranging from a couple of minutes to a few hours. The pulsations are caused by pressure waves, which behave identically to sound waves, that propagate near the surface of the star.
Cepheid Variables and RR Lyrae stars the older and more evolved cousins of delta scuti stars that also pulsate due to pressure waves. These stars are very exciting as they exhibit a pulsation period – luminosity (brightness) relationship, also known as the Leavitt law. This means that we can calculate the distances to these objects by studying the pulsations that we see in their lightcurves. Historically, these stars have helped us to calculate the distance to the Large Magellanic Cloud as well as to the centre of our Galaxy.
Slowly Pulsating B-stars
Slowly Pulsating B-stars (SPB stars) are very similar to Gamma Dor variables but are much more massive (2.5 to ~8 time as massive as the Sun). They are extremely hot with effective temperatures between ~11,000K and ~30,000K and typically show multiple gravity-mode pulsations that range from lasting a couple of hours to several days.
Beta Cephei stars are the high mass (8 to 20 times more massive than the Sun) stars that oscillate due to pressure waves, similar to Delta Scuti stars. The iron interior of these stars reaches extremely high temperatures of 200,000 K. At this temperature, the metal starts to behave strangely, resulting in a build up energy deep within the interior of the star. This causes the star to expand, resulting in an increase in surface area and thus an increase in observed brightness. The expansion of the star, however, uses up the stored energy and eventually it runs out of ‘expansion fuel’. At this point, the star begins to contract again due to gravity, resulting in a decrease in surface area and a decrease in brightness. This cycle repeats resulting in pulsations on time-scales of a couple of hours.
Stars are not simple, and many of them exhibit both pressure and gravity mode pulsations. Those lower mass stars which exhibit both are hybrid delta scuti / gamma dor pulsators, while the higher mass stars which exhibit both are hybrid SPB-Beta Cep pulsators.
Many stars are in binary (double) systems. Heartbeat stars are a class of binary stars that are so eccentric (not circular) that at the point when the stars are closest together, their gravity is so strong that the spherical stars morph into rugby-ball shapes. This increases their visible surface area, and hence increases the total amount of light that we see. Depending on the orientation of the system, we might see either a single brightening, a dip then a brightening, visa versa, or a brightening and eclipse.
We often like to visualise different types of stars on a plot of brightness (luminosity) versus their temperature, an example of which is shown in the figure below.
I love looking through the TESS data and coming across these beautiful light curves of fascinating stars. Maybe some of them even host a planet or two…
RR Lyrae stars are a special type of variable star that changes in brightness due to radial pulsations that increase and decrease the radius of the star . Over the past 3 years, Planet Hunters volunteers on Talk have keenly spotted several previously unknown RR Lyrae stars in the Kepler field, that were nearby neighbors on the CCDs to Kepler targets and were contaminating the photometric aperture used to produce the light curve of the real Kepler target star. You read more about some of these discoveries here. These discoveries have been passed on to collaborators in the Kepler Cepheid & RR Lyrae Working Group who have continued to study those stars including sometimes having the contaminating RR Lyrae added to the Kepler list of targets to get its full light curve.
Katrien Kolenberg who is a member of the Kepler Cepheid & RR Lyrae Working Group, recently wrote a chapter for the conference proceedings of the ’40 Years of Variable Stars: A Celebration of Contributions by Horace A. Smith’ Conference’, and she presented a similarly titled talk at the conference this past May. In the chapter, she gives a summary of the science from the now over 55 RR Lyrae stars known in the Kepler field. She includes a shout out to Planet Hunters to acknowledge the project’s contribution to discovery. Congratulations to all involved in the RR Lyrae discoveries. You can read the chapter from the conference proceedings here.
Although Kepler was designed to find extrasolar planets, the Kepler light curves with their high temporal cadence and measurement precision is a rich data set for studying stellar astrophysics. Although the main goal of Planet Hunters is to search for new extrasolar planets, the Talk discussion tool was designed to enable volunteers to be able to identify other types of potentially interesting variable stars and oddball light curves that we weren’t necessarily looking for with the main classification interface.
With so many eyes looking through the light curves for 160,000 stars on the website, we’re bound to find an interesting star or two, and we have. Planet Hunters has helped discover a new RR Lyrae variable star. This is the second one spotted by Planet Hunters. Just like the first (which was spotted a year ago), this one was spotted by the keen eyes of our volunteers on Talk. It was reported to the Science Team, and Chris contacted the Kepler folks who study these sorts of thing, and it looks like it is indeed a new find. Congratulations to all involved. The RR Lyrae discovery is actually not the Kepler target star, but is nearby and contributing light into the photometric aperture, contaminating the actual Kepler star’s light curve with it’s changing brightness.
RR Lyrae’s are a special type of variable star. The radial pulsations cause the star to expand and contract producing observed changes in the star’s luminosity and subsequently the observed light curve. The American Association of Variable Star Observers (AAVSO) has a nice writeup describing the history and properties of RR Lyraes. Because the pulsations are supposedly simple radial expansion, these stars are often used as standard candles for measuring distances. But there is still a lot to learn about these stars. In particular, the underlying cause of Blazhko modulation, a periodic amplitude and/or phase variation of the pulsations with a period of typically 10-100x the typical pulsation period, that some of the RR Lyraes undergo is still an open question in stellar astrophysics.
This class of variable stars is named for the prototype star, RR Lyrae, first identified to exhibit these oscillations and observed patterns of variation. The original RR Lyrae just so happens to be in the Kepler field as KIC 7198959. There are currently about 40 known RR Lyrae stars in the Kepler field, so this is indeed a rare find. This new find will subsequently be studied by the Kepler Cepheid & RR Lyrae Working Group, and hopefully eventually be included in publication featuring all RR Lyrae stars identified in the Kepler field.
Many stars are not alone, but instead form part of a multi-stellar system of two or more stars that are gravitationally bound together. Even though the prime science goal of TESS is to find exoplanets, it also observes a plethora of eclipsing binaries that allow us to study these systems in more detail.
The light curve of TIC 8153514, observed in Sector 21, shows a sharp eclipse superimposed on top of the signal of a variable star.
If you see eclipsing binaries on PHT you can tag it with #EB or #eclipsingbinary to let others known what you have found!
TESS looks at thousands of stars every night providing us with some of the most beautiful, strange and mysterious light curves. Here is my favourite one of the week: TIC 300446218.
This specific light curve was extracted from the TESS full frame images (FFIs) and it shows multiple dips of different depths. There appear to be two sets of alternating signals which have been highlighted in different colours. The signals highlighted with the red solid lines and the yellow dashed lines are equally spaces but have alternating depths (the yellow signal is deeper) and therefore look like they are caused by an eclipsing binary. Similarly, the signal highlighted by the blue dotted lines and the purple dot-dash lines are also periodic but have different depths. We are, therefore, likely seeing two sets of eclipsing binaries, which, if they are physically close together and tidally locked to one another, could be in a quadruple system!
TIC 300446218 is located close to the ecliptic pole, in the ‘continuous viewing zone’, and was continuously observed by TESS throughout the first year of operation. While the image above only shows a small segment of the light curve, you can see the entire coverage in the figure below.
The full light curve shows that all of the dips disappear after a couple of months and come back again a little while later. This could potentially be due to changes in brightness of a nearby variable star – when the nearby star is bright the dips in TIC 300446218 are diluted and disappear, whereas when the nearby star is faint the dips stand out for us to see. Alternatively, the plane in which the stars are orbiting around one another could be changing inclination by very very small amounts over time, meaning that the stars sometimes cross our line of sight and block a small amount of light, and sometimes don’t. More investigation is needed in order to untangle and understand this fun light curve.
Do you have a favourite light curve of the week?
Think you’ve found a great transit candidate? Can’t wait for us researchers to look into it? Here are a few things that you can do yourself to check whether your candidate could be a real planet. These are the first steps that we would do ourselves, so it’s a great help to us if you have the time or inclination to make a start yourself – and a great opportunity to learn a few cool things in the process. Note you can do as many or as few of the steps on this list as you like – it’s completely up to you!
1. Is it a TOI (Tess Object of Interest)?
TOI is the name used by the TESS team for good planet candidates that they have checked carefully and consider worthy of follow-up observations.
In order to check whether the candidate is a TOI you need to find the TIC number (you can view it by clicking the “i” icon below the subject image in Talk) and check if it appears on the TESS data alerts page: https://archive.stsci.edu/prepds/tess-data-alerts. TIC ID is the first column in the big table. If the candidate is on the TOI list, well done – you have found a candidate that the TESS team have identified as a planet candidate.
If the candidate you found is a TOI you’re doing really well. However, it’s already being looked into by the TESS team, so we won’t duplicate their efforts – we want to focus on objects that they haven’t already found. Before you leave the talk page for that subject though, please tell everyone else what you’ve found – you can say “This is Tess Object of Interest (TOI) XXX” where XXX is the number that appears in the 2nd column on the data alerts table.
2. Is it a TCE (Threshold Crossing Event)?
All of the TESS data are passed through the TESS transit search pipeline, which automatically flags any lightcurves that might contain a planet. TCEs are the raw flagged candidates of this pipeline (prior to any vetting done by the TESS team).
In order to check whether a candidate is a TCE you can download a CSV file, for each sector, where they are all listed:
Alternatively you can check if a given candidate is a TCE using EXOMAST (https://exo.mast.stsci.edu/). On EXOMAST, simply enter “TIC ” followed by the TIC number, and click ‘search’. If the candidate you are looking into is a TCE, you will be taken to a page containing some information about the host star and the potential planetary system.
If the candidate is not a TCE, you will see a notification below the search bar stating “No planet found”.
If you find a TCE, once again, you’re doing really well – it means that you’re as good at finding (some) transits as the pipeline that professional astronomers developed over a number of years!
Please flag such an object as a #TCE on the talk page (if possible including a link to the EXOMAST page for that TCE).
3. It’s a TCE but not TOI?
A candidate that is a TCE but not a TOI is an object that the TESS pipeline flagged, but the TESS team decided wasn’t a good enough planet candidate to be promoted to TOI status. Finding these is really great, not least because – in some cases – we might take a different view to the TESS team and consider them to be likely planet candidates. So if you find a TCE that isn’t a TOI, please let us know by including “@researchers” in your comment on talk. We will get notified automatically and – time permitting – we will look at it more closely.
When vetting the TCEs, the TESS team perform a long list of checks. These tests are designed to weed out instrumental false positive (the signal isn’t real) and astrophysical false positives (the signal is real but isn’t caused by a planet, but something else). The results of these tests are saved in a DV (data validation) report, which they have helpfully made publicly available – so we can use them to understand why the TCE didn’t become a TOI. This is a really quick way to look through candidates and to avoid repeating the hard work that the TESS team have already done. The DV reports are long and complex, and currently a little tricky to access for TCEs that aren’t TOIs, so we are not including instructions on downloading and using DV reports in this post (though we hope to do so at a later date).
Importantly, there are already a few TCE (and not TOI) candidates found by planethunters.org volunteers for which we have examined the DV reports and come to the conclusion that the candidates are promising. This mainly happens because the TESS pipeline requires at least two transits for a detection, so it only searches for transits that repeat with periods up to the duration of a TESS sector (~28 days). If there is only one real transit, it might be missed altogether (this is where you volunteers come in!) or it might be wrongly paired up with an artefact or noise feature somewhere else in the light curve. In that case, the diagnostics in the DV report, which are based on all the transits combined, might be misleading.
4. Create a cutout or movie of the TESS data
There is a fun tool at https://mast.stsci.edu/tesscut/ which allows you to extract a time-series of cutout images around a given target. You can use these to look at what is in the vicinity of the target, or even to make a movie! If the transit is deep enough, you might even see the star “blink” (this can be a fun thing to try out on variable stars or eclipsing binaries too).
Sometimes, what appears to look like a transit is actually due to some weird artefacts, affectionately dubbed “fireflies” or “fireworks” by the TESS team, that sweep through the field of view. These are probably due to scattered light from bright stars or moving objects inside the telescope and camera optics. If you notice that a promising candidate is actually due to such an artefact, please let everyone know on talk!
5. Want to play with the TESS data products yourself?
If you’re really keen and want to examine the TESS data in more detail, you can easily get your hands on them. Go to https://archive.stsci.edu/, enter “TIC” followed by the TIC number of the subject in the search box, and hit “search“. This should bring up a list of datasets stored by MAST (Mikulski Archive for Space Telescopes), including two that will have “TESS” in the “Project” column. The lightcurve is the one that lists the TIC number (rather than “TESS FFI”) under “Target name”.
You can download the data to your computer by clicking on the little floppy disk icon in the corresponding row. You can find more information on the format of these datasets in the TESS Science Data Products Description Document:
What to do with the data when you have it is a long story, far too long for this post… but again, we hope to provide a separate, dedicated article with some examples at a later date.
Please join me in congratulating one of our prolific Planet Hunters, Daryll LaCourse (aka Nighthawk Black), who received the Chambliss Prize for Amateur Astronomy. Woo-hoo! The award was announced at the American Association of Astronomy meeting on January 6th 2016.
Daryll is the second Planet Hunter to receive the Chambliss Amateur Achievement award, which goes to a person not employed in the field of astronomy in a professional capacity, who is resident in North America. The key factor in judging nominations is that the work contributes to the advancement of the science of astronomy.
The citation reads: Daryll LaCourse is a dedicated and talented amateur astronomer who has made significant contributions to exoplanet research as a leading member of the Zooniverse Planet Hunters program. Through painstaking examination and independent reanalysis of Kepler data, he has discovered several new exoplanet candidates, more than 100 previously unknown eclipsing binary systems, and other notable, enigmatic variable stars. He is an energetic and productive collaborator with many professional astronomers. He has coauthored several scientific publications and was lead author on a paper with more than a dozen professional astronomers as co-authors. To quote from one of his letters of support, “If Daryll were a professional astronomer, I would be impressed by the quantity, quality, and creative insight of his work. He is an extraordinary citizen scientist — and highly deserving of the Chambliss award for scientific contributions from amateur astronomers.”
As part of the new Planet Hunters classification interface, the Summary page (see below) suggests some hashtags you could use to label the light curves you’re seeing in Talk and in the Talk comment area on the Summary page. A few people on Talk have asked for a full list, so here’s a handy list of the first set of hashtags suggested by the science team at launch of the new Planet Hunters.
RR Lyrae Star – Pulsating star with periods ~1/2 day
Pulsating star with periods >1 day.
Pulsators – Rapid up and down changes in brightness on the order of a few hours
Eclipsing binary – A star transits another star, often exhibiting V-shape and uneven transit depth
Cataclysmic variable – Cataclysmic variables (cv’s) are a class of stars where the sudden ignition of material on the surface of a white dwarf results in gigantic increase in brightness for several days before returning to natural quiescent state.
Variable star – Change in brightness on timescales greater than 1 day. May be periodic or non-periodic.
Heartbeat star – Two stars get very close together but avoid collision. Their structure changes, and the light curve exhibits a shape like a cadiogram.
glitch – Occasional malfunction of data reduction pipeline.
Planet transit – A planet goes in front of a star and blocks a portion of the star light
Stellar flare – Sudden brightening of a star, often associated with massive material ejection. duration of a few hours. Typically non-periodic.
These listed above are suggested hashtags the science team has come up with. A light curve can definitely be described by more than one hashtag. Also do feel free to use your own hashtags too. There are many more ways to describe and sort the light curves and stars. You can see the most frequent hashtags being used by the Planet Hunters community on the left side of Talk under ‘popular hashtags’
We have made some updates to the labels that you may encounter on Talk. You may might noticed that sometimes below the light curve on its Talk page, there was black text below such as ‘Kepler favorite’ or ‘known eclipsing binary’ for example. In these cases, the light curves where from stars where the Kepler team had already identified what they believed to be an exoplanet transiting or an eclipsing star respectively. In preparation for Quarter 16, we have updated and expanded the list of Talk labels.
Updated Talk Labels
The Kepler team’s planet candidate list from Quarter’s 1-12 is now out, and you’ll find those stars listed as Kepler favorites. Stars that are believed to harbor multiple transiting planets have an additional label, ‘Kepler multiplanet candidate’. The Kepler team has also expanded their list of false positives, where there is a signal the Kepler team spotted in the observations of that star that looks like a transit, but is due to some other astrophysical cause or systematic error. You’ll find those stars labeled as ‘Confirmed Kepler false-positive’. The Kepler eclipsing binary catalog has been updated as well, and a preliminary version of the new catalog was used to identify already known eclipsing binaries.
New Talk Labels
To help with the volunteer-led efforts on Talk to find new planet candidates, we now identify with labels those light curves that are from stars that the Kepler team’s automated detection algorithm identified potential transit signals or Threshold Crossing Events (TCEs) as they are dubbed by the Kepler team. TCEs are not planet candidates, much more vetting and analysis goes into reviewing the TCEs in order to identify the planet candidates among them. You will see the TCEs from the Q1-Q12 observation identified by the Kepler team’s automated routines on Talk with the label ‘Kepler Threshold Crossing Event Candidate’.
Planet Hunters volunteers have been spotting new dwarf novae and RR Lyrae variable stars on Talk. To help with this effort, we have now included labels for both categories. You’ll see ‘Known Dwarf Nova’ and ‘Known RR Lyrae Variable Star’ respectively. Thanks to Daryll (nighthawk_black) for assembling the Dwarf novae list and to Abe (cappella) and Robert Szabo for the RR Lyrae list.
Let’s not forget PH1 b and circumbinary planets, where the planet orbits both stars in a stellar binary. The 6 published circumbinary planets are now labeled in Talk as ‘Confirmed Circumbinary Planet’. So let’s go find another!