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The First Detection of a Starspot During Consecutive Transits of an Extrasolar Planet from the Ground. Steward Observatory Space Grant Conference 2009. Jason Dittmann Undergraduate Advisor: Laird Close Elizabeth Green, Mike Fenwick. Paper submitted to ApJ Letters. What is a Transit?.

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The First Detection of a Starspot During Consecutive Transits of an Extrasolar Planet from the Ground

Steward Observatory

Space Grant Conference 2009

Jason Dittmann

Undergraduate Advisor: Laird Close

Elizabeth Green, Mike Fenwick

Paper submitted to ApJ Letters

tres 1b
  • First extrasolar planet detected using the transit method Alonso et al. (2004)
photometric anomalies
Photometric Anomalies
  • Anomalies have been observed before and after transit
    • Brightening events before ingress and after egress
    • Transit-like events outside of transit
      • Trojans?
photometric anomalies2
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 anomalies3
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 anomalies4
Photometric Anomalies
  • In 2007, ASTR 302 students led by Mike Fenwick also observed strange anomalies in a TrES-1b transit
our observations
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.
  • 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
    • 4 reference stars used for May 15 data
    • 2 of the 4 reference stars used for May 12 due to the patchiness of clouds


  • Transits fit with the method described by Mandel and Agol (2002)
    • RMS noise varied between 2 and 3 mmag
  • Fit the brightening anomaly on May 12 with a sawtooth function.
  • Used that same sawtooth function to find the time of the anomaly on May 15
  • Assume the brightening anomaly is due to TrES-1b occulting a starspot on the surface of TrES-1
    • Therefore, the time of the anomaly in the transits can tell us the rotation period of TrES-1.
relevent formulas
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.

In other words, the distance traveled in the transit time is 2R_p+2R*cos(lat)

rotational period
Rotational Period
  • Using these equations, we find a rotational period of 40.2 +/- 0.1 days
  • But, the subtended angle depends upon the angular tilt of the star
    • Extra factor of cos(λ), where λ = 30 +/- 21 degrees
    • 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
Size of Starspot
  • Approximate spot has 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
Could this be random noise?
  • Probability of noise bursts
    • Binned the residuals of each fit
    • Approximated the residuals as Gaussian distributed
  • Brightening anomaly detected at:
    • 3.2σ May 12 and 2.9σ May 15
  • This is ~1σ above other noise bursts in our data set, so we find it likely that at least one of these events is real.
    • It is not rare for TrES-1 to have a large starspot or starspot group on its surface.
  • The cause of the brightening anomaly during transits of TrES-1b is confirmed as starspots.
  • This is the first instance in which starspots during planetary transits have been observed from the ground.
  • Using geometry, it is possible to accurately calculate the rotational period of the host star and place a lower bound on the size of the spot.
  • The period of TrES-1 is 34.8+4.9-9.5 days
final comments
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.
  • We’d like to thank Trevor Olson and Louis Scuderi for helping to take data during May