HW 4 Q2 (Danny LeBrun)


    In the article by Pravdo and Shaklan, a companion of the star VB 10 is stated to be found through the astrometric method using the Stellar Planet Survey (STEPS) program. They noted that in previous observations of the star it had seemed to contain no planets, however the precisions were too large (1.1-1.5 km/s) in range to be able to file this as fact. The STEPS program observes in R band wavelengths and calculates "reflex motion of a target star around the system center of mass" and then it "is compared to a grid of reference stars in the same field." The reference list for this experiment involved 15 stars and astrometric observations were taken over 9 years. They tested astrometric sensitivity by using two different sets of reference stars in each observation. This negates results being due to the reference stars themselves.
     After obtaining the data, they created a periodogram to analyze VB 10 and the control stars using the Lomb-Scargle method. The significance of a period is determined by having a false alarm probability (FAP) of 1% or lower. This is the criterion for candidate planets. The periodogram found two periods with FAP values smaller than 1% with the smallest being 3E-3% and occurring at a period of 0.74 years. In the two independent axes they combined to calculate the periodogram the FAP was lower than 1% also. Fitting the data to a Keplerian model determined a best fit model with a 0.744-yr period and 6.4 M_J companion. They stated the the small mass and thus low luminosity of VB 10 may discourage RV observations in the future which could completely verify their detection of a planet.
     Moving onto the article by Bean et al, RV observations were made for the star and reported to not have any companions, thus refuting earlier evidence. High precision relative radial velocities were obtained over a span of 0.61 years using high-resolution near-infrared spectra obtained using the CRIRES instrument on the Very Large Telescope with an ammonia gas cell. The precision of the measurements was 10 m/s which is plenty enough to have noticed the proposed 1 km/s variability in the star stated by another article. Unfortunately, this more precise data did not exhibit significant variability at the measurement uncertainties nor any RV variations expected from the presence of an orbiting giant planet as proposed by Pravdo and Shaklan.


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