sigma_xi_presentation.pdf

Schonhut-St
asik (2022) Sigma Xi student research showcase 2022
Underst
anding kepler’s superflaring
G dw
arfs: Investiga
ting whether
superflare beha
vior can be a
ttributed
to unseen, low-mass, companions.
Jessica Schonhut-Stasik, Jennifer Van Saders, Keivan Stassun
Key slide takeaway on each slide

Wha
t is a Stellar Flare?
Schonhut-St
• A stellar flare is a release of magnetic
energy stored near a star spot, during
large eruptions of electromagnetic
radiation.

• A stellar flare on the Sun is known as a
Solar flare.
Routine stellar flare
energies are between
1029 and 1032 ergs.

Wha
t is a Superflare?
Schonhut-St
• Superflares are high energy stellar flares.

• A superflare has never been observed on the
Sun.

• A superflare on the Sun could have potentially
devastating consequences for Earth.

• Superflares have been observed on stars like
the Sun.

• Some other types of stars are theoretically and
observationally known to superflare often (M and
K type)
When the energy of a stellar
flare exceeds 1033 erg, it is
known as a superflare; common
to M and K dwarfs.

superflares on G dw
arfs
and sun-like st
ars
Schonhut-St
• Surveys of mission data from Kepler have found
superflares on G dwarf stars (the Sun is a G2 star), and
‘Sun-like’ stars (G dwarfs with Sun-like properties).

• This raises the following questions:

• Why have we never seen a superflare on the Sun?

• Could there be a dangerous superflare on the Sun in
the future?

• Is the fact that we have not seen a superflare on the
sun a comment about habitability on Earth?

BUT WHA
T IF THE T
ARGET ST
AR IS NOT
ACTUALL
Y FLARING, ONL
Y OBSERVED TO BE
FLARING?
Many superflares have been
observed on G dwarfs and
Sun-like Stars.

Kepler observing superflares
Schonhut-St
• Launched in 2009.

• Initially designed to find
exoplanets around Sun-like stars.

• The light curves created are
perfect for finding superflares!
The Kepler mission is responsible for the
observation of many G dwarf and Sun-like
superflares.
Maehara et al. (2012)

The size of
kepler pixels
Schonhut-St
• Kepler pixels are 4”.

• The ‘aperture’ of the observation
contains multiple pixels.

• If more than one star falls into the
aperture, the flux from that star may
be contained in the final result.
Because Kepler pixels are
big, the flux of lots of stars can
accidentally be observed in a
light curve of a target star.
4”
Target Star
Companion Stars
Schonhut-Stasik et al. (in prep; plot unrelated to this work, shown for reference to Kepler pixels only.)

Wha
t this means
for superflares
Schonhut-St
• If there is a single star, that is not flaring, its
light curve would not have a flare in it.

• But a background flaring star could ‘add’ a
flare to the light curve.

• This could have been misinterpreted as coming
from a target source.
If a second star is flaring, it could look
like the primary target is flaring.
Flux/Time
Flux/Time

LET’S LOOK A
T Shiba
y
ama ET AL. (2013) - S13
Schonhut-St
• In our work we use a model to recreate a model of the S13 sample, but add flaring companion stars.

• This allows us to investigate whether the superflares observed by S13 could have been caused by
superflaring companions.

• We also perform a survey of the stars found to superflare in S13 to determine if they have hidden
companions.

• Finally, we recompute parameters for the S13 superflaring stars to determine if they are G dwarfs at
all!
Shibayama et al. (2013) (S13) found many
superflares on Sun-like and G dwarfs stars
in the Kepler data.

model component
Schonhut-St
We recreated the S13 sample and added
in superflaring companion stars to see if
they would have been detected by
Kepler.
• By comparing our created model to the data
from S13, we can determine how many of the
superflares seen on G dwarfs may have
actually been due to companion stars.
Recreating the Kepler footprint
Recreating the observed Kepler sample
Introducing binaries to this sample
Calculating parameters for the systems
Introducing
fl
ares to the companion stars
Calculating the
fl
are rate

Stellar Popula
tion modelling - 1
Schonhut-St
• TRILEGAL 1.6 population synthesis model.

• Create a full sample of stars for each of the 21
Kepler modules
First, we recreated all the stars in the Kepler
field of view.
The increase in color toward the bottom right
hand side of the plot is consistent with the
increased stellar density toward the galactic
plane.
Schonhut-Stasik et al. (in prep; available in supplemental material.)

Stellar Popula
tion modelling - 2
Schonhut-St
• We cut this sample to match the parameters from a Kepler catalog; Mathur (2017), to recreate
the observed Kepler stars.

• This plot shows the how closely our model result maps to Mathur (2017) - M
17.
Cut the sample to a Kepler catalog to recreate the Kepler observed stars.
Schonhut-Stasik et al. (in prep)

Crea
tion of binaries
Schonhut-St
We assigned binaries (and triples) to the sample
using known statistics:

• Fraction of stars with at least one companion is
40%.

• Assigned orbital periods.

• Assigned mass ratios to the systems.

• Removed twin systems (these would not account for
superflaring companions).
We then chose a subset of the stars to be binary systems, based on known
binary statistics, and allocated parameters, using Moe & Di Stefano (2017).

Calcula
ting system parameters
Schonhut-St
Used known astrophysical quantities to
assign stellar parameters, such as:

• Effective temperature

• log(g)

• Kepler magnitude

• Flux

• Individual masses
We calculated the parameters for the stars in our sample and then cut this down
to contain only the stars we wished to investigate.
We then cut down this sample to contain only G
dwarf target stars, with low-mass M or K dwarf
companions that could superflare.

Crea
ting a sample of
binaries
Schonhut-St
Overview flow chart of how
we created the sample of
binary stars.

M dw
arf flare ra
tes
and energies
Schonhut-St
We modeled the energies of
the superflares and counted
how many could be seen in
the Kepler light curves.
• We modeled the number of flares that would
happen per day (Davenport et al. (2019)), and
scaled this up to the number of days of observation.

• We modeled the energy and relative flux of the
superflares to determine if they would be detected
in the light curves of the primary star.
Davenport et al. (2019)

model flare ra
te
Schonhut-St
To compare our results to S13, we created a ‘flare rate’ equation to
compare the numbers directly.
• The units of this equation is superflares per stars in the sample per century.

• Flare rate from our model = 1.17 superflares/stars in the sample/century.

• Our calculation of the S13 flare rate = 1.67 superflares/stars in the sample/century.
Scaling constant to give time in our equation
1.17 / 1.67 = 0.700; implies that 70% of the S13 superflares could be explained by
nearby, low-mass, superflaring companions.
NB: These are preliminary results and are
subject to change before publica
tion.

Modeling superFlare Ra
te
Schonhut-St
Overview flow chart of how
we modeled superflares and
counted them.

Survey component
Schonhut-St
We surveyed the stars in the
S13 superflaring sample to
see if we could find
companions
• We looked for companion stars in Keck, Gaia,
and UKIRT archival data.

• We also updated parameters of stars to make
sure they are actually G dwarfs or Sun-like stars.
Keck, Maunakea Hawai’i
UKIRT, Maunakea Hawai’i Gaia, Space

Resol
ved St
ars
Schonhut-St
• We searched UKIRT archival data for
resolved companions.

• The companions found could fall within
the Kepler lightcurve of the target star.

• We reinforced these findings with a
search of the Gaia archive.
We searched for companions that
could fall in the Kepler target pixel.
Target Star
Companion star
inside target pixel
Companion star
outside target pixel
ID of target
star
Schonhut-Stasik et al. (in prep.)

Gaia survey - Ruwe
Schonhut-St
• A companion star may be too close to the target star
to see in images. We found these using the RUWE.

• RUWE = re-nomalized unit weight error.

• RUWE is a Gaia parameter that can be indicative of
a binary star.

• A value of > 1.1 indicates a binary.
We used the RUWE metric to find
companions too close to resolve in images.
Belokurov et al. (2020)
RUWE we would expect from single
stars.
Stars that exist in Gaia with higher
RUWE.

Schonhut-St
survey component results - Companions
• Our survey finds 63 +/- 1
1% of the S13 sample have
at least one spatially resolved companion.

• This supports our model conclusion that 70% of the
superflares in the S13 sample may have come from
companions.
Our survey component supports our model result.

Upda
ted Parameters
Schonhut-St
• Checked more recent Kepler catalogs to see if stars had
updated effective temperatures and log(g). Those that
did could have been mischaracterized in the S13 study.

• Looked for updated rotation periods of stars. If these
stars were rotating faster that previously expected, it may
not be unusual for them to superflare - but they cannot
be similar to the Sun.

• Looked for updated evolutionary status’ of stars. Some
were originally misclassified as dwarfs when they were
actually subgiants.
We used updated Kepler
stellar parameter catalogs
to determine if the original
sample had any
mischaracterized stars. i.e.,
that were not actually G
dwarfs.

Assessing the S13 Sample
Schonhut-St
• We considered the original S13 sample in terms of
rotation period.

• The Sun rotates with a period of around 25 days.
This is considered a slowly-rotating star.

• The plot shows that many of the stars that S13
consider Sun-like, actually rotate much quicker.

• Faster rotating stars are more likely to display
superflares, but cannot be compared in the same
way to the Sun.
All G Dwarfs
Sun-like stars
The S13 sample do not
rotate with Sun-like periods.

Preliminary Results
Schonhut-St
1. Our model flare rate is 1.17 [superflares/total stars/century] and the flare rate for S13 is
1.67 [superflares/total stars/century]. This suggests that 70% of the observed superflares in
the S13 sample could be explained by superflaring companions.

2. Our survey finds 63 +/- 1
1% of the S13 sample have at least one spatially resolved
companion.

3. 77% of the stars in the S13 sample, rotate with <10 days, implying their youth and activity
could be the cause of their superflares.
NB: These are preliminary results and are
subject to change before publica
tion.

sigma_xi_presentation.pdf

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