First Detection of a Starspot During Transits of an Extrasolar

Planet from the Ground

Steward Observatory

Undergraduate Symposium

Jason Dittmann

Undergraduate Advisor: Laird Close

Elizabeth Green, Mike Fenwick

Paper submitted to ApJ Letters

What is a Transit?

TrES-1b

• First extrasolar planet detected using the transit method Alonso et al. (2004)

Photometric Anomalies

Photometric Anomalies• Many observers

have noticed brightening anomalies during transits of TrES-1b. (Price et al. 2004)

• Hubble ACS data has shown brightening anomalies during transit (Rabus et al. (2009)

Photometric Anomalies

• Winn et. al (2007) observed 3 consecutive transits in the Z band with the 1.2 meter telescope at FLWO.

• No evidence for photometric anomalies

• However, if these anomalies are starspots, they would be less sensitive to them in the Z-band

Photometric Anomalies• In 2007, ASTR 302 students also observed strange anomalies

in a TrES-1b transit

Our Observations• Observations taken at

the 61” Kuiper Telescope on Mt Lemon with the Mont4k CCD.

• 2 consecutive transits (May 12 and May 15, 2008 UT)

• Images taken in the R-band

• Light clouds on May 12, photometric on May 15.

Reductions

• 2σ clipping applied to reference stars

• Data points for TrES-1 were not clipped

• TrES-1 flux normalized to the weighted average of the reference stars

TrES-1

Analysis

• Transits fit with the method described by Mandel and Agol (2002)– RMS noise varied between 2 and 3 mmag– Excluding the anomaly data points

• Therefore, the time of the anomaly in the transits can tell us the rotation period of TrES-1.

Geometry

Relevent Formulas

The factor of R_p / R_* cos(lat) corrects for the fact that ingress is when the leading limb of the planet crosses the star while egress is when the trailing limb of the planet leaves the star.

Rotational Period

• Prot = 40.2 +/- 0.1 days

• But, the subtended angle depends upon the angular tilt of the star– Yields P = 34.8+4.9

-9.5 days

– Consistent with previous measurement of 33+25-16

days

• If TrES-1 had made 2 full rotations between observations, the period would be 2.84 days– Unlikely for an old K0V star

Size of Starspot

• Approximate spot as 100% black

• Peak Brightening ~5.4 mmag

• Depth of transit ~25 mmag

• The ratio of the brightening to the depth is equal to the ratio of the spot size to the size of the planet.

• Rspot >~ 6 Rearth

Could this be random noise?

• Brightening anomaly detected at:– 3.2σ May 12 and 2.9σ May 15

• This is ~1σ above other noise bursts in our data set– It is not rare for TrES-1 to have a large

starspot or starspot group on its surface.

Results

• The cause of the brightening anomaly during transits of TrES-1b is confirmed as large starspot(s).

• This is the first instance in which starspots during planetary transits have been observed from the ground.

• The period of TrES-1 is 34.8+4.9-9.5 days

Final Comments• Starspots are a new source of noise for

planetary transit data– Understanding and modeling them will be

of use to the Kepler mission in finding extrasolar Earths.

• This result was encouraging enough to try to observe a transit whose depth is equal to the anomaly height itself!

And it worked!

Thanks!

Questions?

We’d like to thank Trevor Olson and Louis Scuderi for helping to take data during May