Investigating the physical properties of transiting hot Jupiters with the 1.5-m Kuiper Telescope
Jake D. Turner Robin M. Leiter Lauren I. Biddle Kyle A. Pearson Kevin K. Hardegree-Ullman Robert M. Thompson Johanna K. Teske Ian T. Cates Kendall L. Cook Michael P. Berube Megan N. Nieberding Christen K. Jones Brandon Raphael Spencer Wallace Zachary T. Watson Robert E. Johnson




The Asymmetric Colonel Blotto Game
    Yifan Zhu
    December 30, 2023
==================================





We present new photometric data of 11 hot Jupiter transiting exoplanets (CoRoT-12b, HAT-P-5b, HAT-P-12b, HAT-P-33b, HAT-P-37b, WASP-2b, WASP-24b, WASP-60b, WASP-80b, WASP-103b, XO-3b) in order to update their planetary parameters and to constrain information about their atmospheres. These observations of CoRoT-12b, HAT-P-37b and WASP-60b are the first follow-up data since their discovery. Additionally, the first near-UV transits of WASP-80b and WASP-103b are presented. We compare the results of our analysis with previous work to search for transit timing variations (TTVs) and a wavelength dependence in the transit depth. TTVs may be evidence of a third body in the system and variations in planetary radius with wavelength can help constrain the properties of the exoplanet's atmosphere. For WASP-103b and XO-3b, we find a possible variation in the transit depths that may be evidence of scattering in their atmospheres. The B-band transit depth of HAT-P-37b is found to be smaller than its near-IR transit depth and such a variation may indicate TiO/VO absorption. These variations are detected from 2-4.6σ, so follow-up observations are needed to confirm these results. Additionally, a flat spectrum across optical wavelengths is found for 5 of the planets (HAT-P-5b, HAT-P-12b, WASP-2b, WASP-24b, WASP-80b), suggestive that clouds may be present in their atmospheres. We calculate a refined orbital period and ephemeris for all the targets, which will help with future observations. No TTVs are seen in our analysis with the exception of WASP-80b and follow-up observations are needed to confirm this possible detection.




 planets and satellites: atmospheres – techniques:
photometric – planet-star interactions 







§ INTRODUCTION
 


To date, over 3400 exoplanets have been discovered (NASA Exoplanet Archive; ) and most of these planets have been found using the transit method (e.g. ; ) in large-scale transit surveys such as Kepler (), K2 (), WASP (; ) and CoRoT (; ). Transiting exoplanet systems (TEPs) are of great interest because their radius can be directly measured in relation to their star with photometric observations (; ). With the addition of spectroscopic and radial velocity measurements, many physical properties of TEP systems (mass, radius, semi-major axis, gravity, temperature, eccentricity, orbital period) can be directly measured (e.g., ). Additionally, multiple-band photometry of a TEP system can be used to constrain the composition of an exoplanet's atmosphere (; ; ; ). The absorption properties of different species in a planetary atmosphere vary with wavelength, causing an observable variation in the planet's radius. Photometric light curve analysis can also be used to search for transit timing variations (TTVs). TTVs can indicate additional bodies in a TEP system or an unstable orbit caused by tidal forces from the star (e.g., ; ; ). 

In this work, we present new ground-based photometric data of 11 confirmed transiting hot Jupiter exoplanets. We describe and perform TEP modeling techniques (Section <ref>–<ref>) to determine the orbital and physical parameters of each system, and compare our results with previous published results to confirm and improve the planetary parameters (Section <ref>–<ref>). For each system, we combine our results with previous work to search for a variation in planetary radius with wavelength (Section <ref>), which could indicate Rayleigh scattering, the presence of an absorptive atmosphere, or clouds. Finally, we combine our mid-transit data with previous observations to recalculate each system's orbital period and search for TTVs.





§ OBSERVATIONS AND DATA REDUCTION
 
All the observations were performed at the University of Arizona's Steward Observatory 1.55-m Kuiper Telescope on Mt. Bigelow near Tucson, Arizona. The Mont4k CCD has a field of view of 9.7'×9.7' and contains a 4096×4096 pixel sensor. The CCD is binned 3×3 to achieve a resolution of 0.43/pixel and binning reduces the read-out time to ∼10 s. Our observations were taken with the Bessell U (303–417 nm), Harris B (360–500 nm), and Harris R (550–900 nm) photometric band filters. To ensure accurate timing in these observations, the clocks were synchronized with a GPS every few seconds. In all the data sets, the average shift in the centroid of our targets is less than 0.6 pixels (0.26) due to excellent autoguiding (the maximum is 3.4 pixels). This telescope has been used extensively in exoplanet transit studies (; ; ; ; ; ; ; ). A summary of all our observations are displayed in Table <ref>.

To reduce the data and create the light curves we use the reduction pipeline [https://sites.google.com/a/email.arizona.edu/kyle-pearson/exodrpl] <cit.>. Each of our images are bias-subtracted and flat-fielded with 10 biases and flats. To produce the light curve for each observation we perform aperture photometry (using  in the  package) by measuring the flux from our target star as well as the flux from 8 different reference stars with 110 different circular aperture radii. The aperture radii sizes we explore are different for every observation due to changes in seeing conditions. For the analysis, a constant sky annulus for every night of observation of each target is chosen (a different sky annulus is used depending on the seeing and the crowdedness of the target field) to measure the brightness of the sky during the observations. We reduce the risk of contamination by making sure no stray light from the target star or other nearby stars falls in the chosen aperture. A synthetic reference light curve is produced by averaging the light curves from our reference stars. The final light curve of each date is normalized by dividing by this synthetic light curve to correct for any systematic differences from atmospheric variations (i.e. airmass) throughout the night. Every combination of reference stars and aperture radii are considered and we systematically choose the best aperture and reference stars by minimizing the scatter in the Out-of-Transit (OoT) data points. The 1σ error bars on the data points include the readout noise, flat-fielding errors, and Poisson noise. The final light curves are presented in Figs. <ref>–<ref>. The data points of all our transits are available in electronic form (see Table <ref>). For all the transits, the OoT baselines have a photometric root-mean-squared (RMS) value between 1.13 and 7.76 millimagnitude (mmag).



















 




§ LIGHT CURVE ANALYSIS


To find the best-fit to the light curves we use the EXOplanet MOdeling Package (; ; )[ is used in the analysis and is available on Github at https://github.com/astrojake/EXOMOPhttps://github.com/astrojake/EXOMOP ], which utilizes the analytic equations of <cit.> to generate a model transit. For a complete description of  see <cit.> and <cit.>. The χ^2-fitting statistic for the model light curve used in  is: 

    χ^2 = ∑_i=1^N_pts[  f_i(  )  - f_i()/σ_i()]^2

where N_pts is the total number of data points (Table <ref>), f_i() is the observed flux at time i, σ_i() is the error in the observed flux, and f_i() is the calculated model flux.

 uses the following procedure to find a best-fit to the data. A Levenberg-Marquardt (LM) non-linear least squares minimization (; ; ) is performed on the data and a bootstrap Monte Carlo technique () is used to calculate robust errors of the LM fitted parameters. Additionally, a Differential Evolution Markov Chain Monte Carlo (DE-MCMC; ; ) analysis is used to model the data. The fitted parameters that have the highest error bars from either the LM or DE-MCMC best-fitting model are used in the analysis. In every case both models find results within 1σ of each other. Additionally,  uses the residual permutation (rosary bead; ), time-averaging (), and wavelet () methods to assess the importance of red noise in both fitting methods. Not accounting for red noise in the data underestimates the fitted parameters (; ). In order to be conservative, the red noise method that produces the largest errors is used to inflate the errors in the fitted parameters. Finally, in order to compensate for underestimated observational errors we multiply the error bars of the fitted parameters by √(χ^2_r) when the reduced chi-squared (χ^2_r) of the data (Table <ref>) is greater than unity (e.g. ; ;  ;  ; ; ). 

 uses the Bayesian Information Criterion (BIC; ) to assess over-fitting of the data. The BIC is defined as 

    BIC =  χ^2 + k ln(N_pts),

where χ^2 is calculated for the best-fitting model (equation <ref>) and k is the number of free parameters (Table <ref>) in the model fit [f_i()]. The power of the BIC is the penalty for a higher number of fitted model parameters, making it a robust way to compare different best-fit models. The preferred model is the one that produces the lowest BIC value. 

Each transit is modeled with  using 10000 iterations for the LM model and 20 chains and 20^6 links for the DE-MCMC model. The Gelman-Rubin statistic () is used to ensure chain convergence () in the MCMC model. During the analysis of each transit the mid-transit time (T_c), planet-to-star radius (R_p /R_*), scaled semi-major axis (a/R_*), and inclination (i) are set as free parameters. The previously published values for a/R_*, i, and a/R_* are used as priors for the LM model (Table <ref>). The results of the LM fit are used as the prior for the DE-MCMC. The eccentricity (e), argument of periastron (ω), and period (P_p) of each of the planets are fixed (see Table <ref> for their values) in the analysis because these parameters have minimal effect on the overall shape of the light curve. The linear and quadratic limb darkening coefficients in each filter are taken from <cit.> and interpolated to the stellar parameters of the host stars (see Table <ref>) using the  applet[http://astroutils.astronomy.ohio-state.edu/exofast/limbdark.shtml]<cit.>. In addition, a linear or quadratic least squares fit is modeled to the OoT baseline simultaneously with the <cit.> model. The BIC is used to determine whether to include any baseline fit in the best-fit model and the baseline with the lowest BIC value is always chosen.



The light curve parameters obtained from the  analysis and the derived transit durations are summarized in Table <ref>. The modeled light curves can be found in Figs. <ref>–<ref> and the physical parameters for our targets are derived as outlined in Section <ref> (Tables <ref>–<ref>). A thorough description of the modeling and results of each system can be found in Section <ref>. 









 




§ PHYSICAL PROPERTIES OF THE SYSTEMS

 


We use the results of our light curve modeling with  combined with other measurements in the literature to calculate the planetary mass (e.g. ; ), radius, density, surface gravity (e.g. ), modified equilibrium temperature (e.g. ), Safronov number (e.g. ; ), and atmospheric scale height (e.g. ; ). An updated period and ephemeris is also calculated and is described in detail in Section <ref>. To calculate the physical parameters we use the values from the modeling (P_p, R_p/R_∗, i, a/R_∗) and for the orbital (e) and host star parameters (radial velocity amplitude, mass, radius, equilibrium temperature) we use the values found in the literature. When calculating the scale height, the mean molecular weight in the planet's atmosphere was set to 2.3 assuming a H/He-dominated atmosphere (). The physical parameters of all our systems can be found in Tables <ref>–<ref>.



































 






 







 §.§ Period Determination

By combining our mid-transit times found using  with previously published mid-transit times, we can refine the orbital period of the targets. When necessary, the mid-transit times were transformed from HJD, which is based on UTC time, into BJD, which is based on Barycentric Dynamical Time (TDB), using the online converter[http://astroutils.astronomy.ohio-state.edu/time/hjd2bjd.html] by <cit.>. A refined ephemeris for each target is found by performing a weighted linear least-squares analysis using the following equation:



    T_c = T_c(0) + P_p×E,
 
where T_c(0) is the mid-transit time at the discovery epoch measured in BJD_TDB, P_p is the orbital period of the target and E is the integer number of cycles after their discovery paper. See Tables <ref>-<ref> for an updated T_c and P_b for each system.

For every system, we also made observation minus calculation mid-transit time (O-C) plots in order to search for any TTVs due to other bodies in the system. We used the derived period and ephemeris found in Tables <ref>-<ref> and Equation <ref> for the calculated mid-transit times. The O-C plots can be found in Figs <ref>–<ref>. The transit timing analysis for all our targets can be found in Table <ref> (the entire table can be found online). We do not observe any significant TTVs in our data with the exception of a 3.8σ deviation for WASP-80b for our observed transit. Since the possible TTV is only one data point and may be caused by an unknown systematic error, more observations of WASP-80b are needed to confirm this result.








 
 
 
 




§ INDIVIDUAL SYSTEMS




 §.§ CoRoT-12b

CoRoT-12b was discovered by the CoRoT satellite () and was confirmed by follow-up photometry and radial-velocity measurements (). CoRoT-12b is an inflated hot Jupiter with a low density that is well predicted by standard models () for irradiated planets ().

We observed a transit of CoRoT-12b on 2013 February 15 with the Harris R filter (Fig. <ref>). We find a R_p/R_∗ value 4.6σ greater than the discovery value. Our derived physical parameters are in good agreement with <cit.>. We find a planetary radius within 1.3σ of the previously calculated value and a planetary mass within 1σ (Tables <ref> and <ref>). 










 §.§ HAT-P-5b

HAT-P-5b is a hot Jupiter discovered by the HATNet project that orbits a slightly metal-rich star (). Follow-up multi-color transit observations of HAT-P-5b by <cit.> confirmed the existence of the planet and searched for a variation in planetary radius with wavelength. A significantly larger radius was found in the U-band than expected from Rayleigh scattering alone, which the authors suggest may be due to an unknown systematic error.

We observed a transit of HAT-P-5b on 2015 June 6 with the Bessell U filter (Fig. <ref>). Our derived physical parameters are in agreement with previous literature (Tables <ref> and <ref>). We derive a U-band radius consistent with a weighted average of radii taken from 350-733 nm within 1σ (Table <ref>; Fig <ref>). The error on our U-band observation is too large to determine if the observation by <cit.> in the same band may have an unknown systematic error (as suggested by them). Our calculated period is in good agreement with the value found by <cit.> with a similar uncertainty. 






















 §.§ HAT-P-12b

HAT-P-12b is a low density, sub-Saturn mass planet discovered by the HAT survey (). Multiple photometric studies have further refined the system's parameters and searched for TTVs (; ; ; ; ; ; ). <cit.> find a strong optical scattering slope from blue to near-IR wavelengths using Hubble Space Telescope and Spitzer Space Telescope transmission spectrum data. 










We observed a transit of HAT-P-12b on 2014 January 19 using the Harris B filter (Fig. <ref>). We derive an optical R_p/R_∗ within 1σ of previously derived radii at optical wavelengths (Table <ref>; Fig <ref>). These results are consistent with the planet having high clouds in its atmosphere (e.g. ; ) and the finding by <cit.> that HAT-P-12b has a cloudy atmosphere. We also find a period similar to <cit.>. 





















 §.§ HAT-P-33b

HAT-P-33b is an inflated hot Jupiter orbiting a high-jitter star <cit.>. The high-jitter is believed to be caused by convective inhomogeneities in the host star (; ). The planetary radius and mass, which both depend on eccentricity, and the stellar parameters are not well constrained due to the large jitter (20  m  s^-1). HAT-P-33b's radius is either 1.7 or 1.8 R_Jup assuming a circular or eccentric orbit, respectively. The first follow-up observations by the Transiting Exoplanet Monitoring Project (TEMP) of HAT-P-33b confirmed the discovery parameters and detected no signs of TTVs ().

We observed one transit of HAT-P-33b on 2012 April 6 with the Harris R filter (Fig. <ref>). We find a R-band R_p/R_∗ value that is larger by 3.4σ from the discovery R_p/R_∗ (Table <ref>). Follow-up observations are need to determine the cause of this discrepancy.





 §.§ HAT-P-37b

HAT-P-37b was identified by the HATNet survey and was confirmed by high-resolution spectroscopy and further photometric observations (). HAT-P-37b is a hot Jupiter with a planetary mass of 1.169±0.103 M_Jup, a radius of 1.178±0.077 R_Jup, and a period of 2.797436±0.000007 d. Additional follow-up observations by <cit.> confirmed these planetary parameters.

We obtained two transits of HAT-P-37b on 2015 July 1 with the Harris B and R filters (Fig. <ref>). We derive an R_p/R_∗ for each filter that differ by 1.7σ, with a larger radius in the R band (Table <ref>; Fig <ref>). The B-band R_p/R_∗ is smaller by 2.85σ from the near-IR R_p/R_∗ (Table <ref>; ). Our derived R-band R_p/R_∗ value agrees within 1σ of the Sloan i band value obtained by <cit.>. Near-UV observations are needed to determine if the slope between the B and R filters is real or an unknown systematic in the data. Our other derived physical parameters agree with previous literature to within 1σ (Tables <ref> and <ref>). We also calculate a refined period with a factor of 6 decrease in error. 









 §.§ WASP-2b

WASP-2b is a short-period hot Jupiter discovered by the WASP survey and confirmed by radial-velocity measurements taken with the SOPHIE spectrograph (). Extensive photometry and radial velocity measurements have been performed on WASP-2b, further refining its system parameters (; ; ; ; ; ; ; ; ).

We observed WASP-2b on 2014 June 14 with the Harris B filter (Fig. <ref>). Our derived physical parameters and transit depth agree with previous literature to within 1σ and we calculate a period with a factor of 2 decrease in error (Tables <ref> and <ref>). 



























 §.§ WASP-24b

WASP-24b is a hot Jupiter detected by WASP and confirmed by radial velocity measurements and additional photometric observations (). Further photometric studies calculated improved system parameters () and radial velocity measurements were used to determine that the planet exhibits a symmetrical Rossiter-McLaughlin effect, indicating a prograde, well-aligned orbit ().

Our observations of WASP-24b took place on 2012 March 23 and 2012 April 6 (Fig. <ref>). We obtained two transits with the Harris R filter and each transit was modeled separately. The R_p/R_∗ of both dates overlap each other within 1σ. We then found the weighted average of the light curve parameters before deriving the physical parameters. Our weighted average R_p/R_∗ disagrees with previous R-band observations () by 4σ (Table <ref>; Fig <ref>). The cause of this difference is unknown but future observations can put better constraints on transit depth and solve this discrepancy. Our other derived parameters generally agree with previous results except for our planetary radius and equilibrium temperature, which differ from <cit.> by 1.4σ and 1.9σ, respectively (Tables <ref> and <ref>). We calculate a new period with a factor of 2.6 decrease in error (Table <ref>).








 §.§ WASP-60b

WASP-60b was identified by WASP-North and was confirmed by radial-velocity measurements and follow-up photometry (). WASP-60b is an unexpectedly compact planet orbiting a metal-poor star.

We observed a transit of WASP-60b with the Harris B filter on 2012 December 1 (Fig. <ref>). This observation is the first follow-up light curve of WASP-60b. During observations, the automatic guider briefly failed, resulting in a hole in the transit light curve. Despite this, we are able to derive parameters that agree with previous literature to within 1σ (Tables <ref> and <ref>). We find a B-band R_p/R_∗ value 1.3σ greater than the discovery R_p/R_∗.  







 §.§ WASP-80b

WASP-80b is a warm Saturn/hot Jupiter (M_p= 0.55±0.04M_jup) with one of the largest transit depths (0.17126±0.00031) discovered so far (). Multiple photometric studies have been done at various wavelengths to refine WASP-80b's planetary parameters (; ; ; ; ). The planet has a transmission spectrum consistent with thick clouds and atmospheric haze ().

We observed WASP-80b on 2014 June 16 with the Bessell U filter, obtaining one transit (Fig. <ref>). Inclement weather conditions caused our guider to briefly fail, resulting in a hole in the transit light curve. We derive physical parameters that closely agree with previous literature and also calculate a slightly a refined period with a factor of 2 decrease in error (Tables <ref> and <ref>). Our observations possibly detect a TTV compared to previous work by 3.7σ (Section <ref>), however, further observations of WASP-80b are needed in order to confirm this result.  

















 §.§ WASP-103b
 
WASP-103b is a hot Jupiter detected by the WASP survey with a mass of 1.49±0.09 M_jup, short period planet (P_p = 0.925542±0.000019  d), and has an orbital radius only 20% larger than its Roche radius (). It was found that there is a faint, cool, and nearby (with a sky-projected separation of 0.242±0.016 arcsec) companion star of WASP-103 (; ). Further photometric observations were made to refine WASP-103b's planetary parameters and ephemeris (; ; ). A comparison of observed planetary radius at different wavelengths found a larger radius at bluer optical wavelengths, but <cit.> state that Rayleigh scattering cannot be the main cause even when including the contamination of the nearby companion star. 

We observed WASP-103b on 2015 June 3 with the Bessell U filter (Fig. <ref>). We derive a dilution-corrected near-UV ( R_p/R_∗)_cor that differs from the discovery value by 2.1σ. Our other calculated parameters agree with previous literature to within 1σ and our calculated period closely agrees with the period found by <cit.> (Tables <ref> and <ref>). A variation in R_p/R_∗ is found from the ultraviolet to the near-infrared wavelengths (Table <ref>; Fig <ref>) consistent with that found by <cit.>. 

We correct for the dilution due to the companion star being in our aperture using the procedure described below (this procedure is similar to that done by ). (1) The light curve is modeled with EXOMOP and we find an uncorrected transit depth of (R_p/R_∗)_uncor = 0.1174±0.0016. (2) Theoretical spectra of both stars is produced using ATLAS9-ODFNEW (). For WASP-103 we use T_eff = 6110 K and M_star = 1.22  M_ () and for the companion star we use T_eff = 4405 K () and M_star = 0.721  M_ (). Additionally, in order to scale the spectrum correctly we use the mass-lumnosity relation L=L_(M/M_)^4 for stars between 0.5 and 2 M_. (3) The ATLAS9-ODFNEW model spectra is convolved with the bandpass of the Bessell U filter (). (4) The corrected transit depth, ( R_p/R_∗)_cor, is found using the equation ()




    (R_p/R_∗)_cor    = (R_p/R_∗)_uncor√(F_tot/F_2),

where F_tot is the total flux of both stars and F_2 is the flux from the companion star. In <cit.> the error of the photometric light curve dominated the error calculation of their corrected transit depth and therefore we also use our photometric error bars for the error in the (R_p/R_∗)_cor. Using this procedure we find a (R_p/R_∗)_cor = 0.1181±0.0016. 





















 §.§ XO-3b

XO-3b is a massive planet (11.79±0.59 M_Jup) with a large eccentricity (0.26± 0.017) detected by the XO survey (). Further photometric observations have refined the system's parameters (; ; ; ) and <cit.> found that XO-3's spin axis is misaligned with XO-3b's rotation axis.



We observed a transit of XO-3b on 2012 November 30 with the Harris B filter (Fig. <ref>). We derive physical parameters that are in agreement with previous literature (Tables <ref> and <ref>). Our calculated R_p/R_∗ is 2σ larger than the V-band R_p/R_∗ found by <cit.>. We calculate a refined period with an error decreased by a factor of 13 from the value found by <cit.>. A non-flat spectrum for R_p/R_∗ is found for XO-3b (Table <ref>; Fig <ref>).



















§ DISCUSSION



 §.§ Wavelength dependence on the transit depth
 


We find a constant transit depth across optical wavelengths for the TEPs HAT-P-5b, HAT-P-12b, WASP-2b, WASP-24b, and WASP-80b (Fig <ref>, Table <ref>). A lack of variation in radius with wavelength could suggest these planets (HAT-P-5b, HAT-P-12b, WASP-2b,  WASP-80b) have clouds/hazes in their upper atmospheres (e.g. ; ; ; ; ) or they have an isothermal pressure-temperature profile <cit.>. <cit.> also do not detect a significant variation in WASP-80b's transit depth with wavelength, and <cit.> finds a relatively flat spectrum of planetary radii for HAT-P-5b with the exception of their observed radius in the U-band (which they suspect is caused by systematic error in their U-band photometry). A flat spectrum for WASP-24b is also found with the exception of one value. Our R-band R_p/R_∗ found for WASP-24b differs by 4σ from the previously calculated R_p/R_∗ () for that same band. The cause of this is unclear and future observations are needed to investigate. Our results are consistent with other transiting exoplanet observations having a flat spectrum in optical wavelengths (i.e. TrES-3b, ;  GJ 1214b, ; ; WASP-29b, ; ; HAT-P-19b, ; HAT-P-1b, HAT-P-13b, HAT-P-16b, HAT-P-22b, TrES-2b, WASP-33b, WASP-44b, WASP-48b, WASP-77Ab, ). 

We find variations in the transit depth with wavelength for CoRoT-12b, HAT-P-33b, HAT-P-37b, WASP-103b, and XO-3b (Fig <ref>-<ref>, Table <ref>), which could indicate scattering (i.e. due to aerosols or Rayleigh scattering) or absorption in their atmospheres (e.g. ; ). Our observation of HAT-P-37b exhibits a smaller transit depth in B-band than the red/near-IR value. Such a variation has only been seen in a recent paper by <cit.> where they observe a smaller B-band transit depth than optical in WASP-121b. <cit.> believe a possible cause of such a variation is TiO/VO absorption and this may also be the cause of the transit depth variations seen in HAT-P-37b. However, more theoretic modeling is needed to confirm that TiO/VO is in fact the opacity source. Additionally, a smaller near-UV radius was recently observed in the hot Jupiter WASP-1b (), however, these observations did not observe in the B-band. Future near-UV and blue-band observations are needed for WASP-103b and XO-3b to determine whether the scattering in their atmospheres is due to Rayleigh scattering (; ; ; ) since these bands are the only optical wavelengths not affected by strong spectral features. The radius variations in WASP-103b show a consistently larger transit depth in the near-UV and blue than the rest of the optical (this varation is still present when corrected for dilution due the companion star). Such a radius variation may indicate a change in particle size at different altitudes of the planetary atmosphere (e.g. ). We find a larger R-band transit depth in HAT-P-33b and CoRoT-12b than their discovery transit depths. Since the R-filter encompasses the Hα line (656.281 nm), our observation could be an indication of atmospheric escape such as that observed in the atmospheres of HD 189733b (; ; ; ; ) and HD 209458b <cit.> and predicted (e.g. ; ). Follow-up photometry and high-resolution spectroscopy observations are encouraged to confirm all the transit depth variations. These results also agree with observations of other exoplanets not having a flat spectrum (i.e. HD 209458b, ; HAT-P-5b, ; GJ 3470b, ; Qatar-2, ; WASP-17b, WASP-39b, HAT-P-1b, WASP-31b, HAT-P-12b, HD189733b, WASP-6b, ; CoRoT-1b, TrES-4b, WASP-1b, WASP-12b, WASP-36b, ).




For illustration, the observed R_p/R_∗ differences with wavelength for each target (Table <ref>) are compared to theoretical predictions () for a model planetary atmosphere (Figure <ref>–<ref>). The models used are calculated for planets with a 1 M_Jup, g_p = 25 m s^-1 or g_p = 10 m s^-1, base radius of 1.25 R_Jup at 10 bar, T_eq closest to the measured value for each exoplanet (with model choices of 500, 750, 1000, 1250, 1500, 1750, 2000, 2500 K), and solar metallicity. To provide a best fit to the spectral changes a vertical offset is applied to the model. This comparison is helpful as it illustrates the size of observed variation compared to what the theoretical models predict. However, radiative transfer models calculated for each exoplanet individually are needed to fully understand their transmission spectra.

 Finally, no signs of asymmetric transits are seen in the near-UV light curves of HAT-P-5b, WASP-80b, and WASP-103b. This result is consistent with ground-based near-UV observations of 19 other transiting exoplanets (; ; ; ; ; ; ) that show no evidence of asymmetric transits. Additionally, theoretical modeling by <cit.> using the  plasma simulation code showed that asymmetric transits cannot be produced in the broad-band near-UV band regardless of the assumed physical phenomena that could cause absorption (e.g. ; ; ; ; ).   




  §.§.§ Variability in the host stars
 





One of the major assumptions in our interpretation that the planetary atmosphere is the cause of the transit depth variations is that the brightness of the host stars have minimal variability due to stellar activity. The presence of star spots and stellar activity can produce variations in the observed transit depth (e.g. ; ; ; ; ). This effect is stronger in the near-UV and blue and can mimic a Rayleigh scattering signature (e.g. ; ). Additionally, no obvious star spot crossing is seen in our data (Figs. <ref>-<ref>) with the possible exception of HAT-P-37b (see below).

We estimate how much the transit depth may change due to unocculted spots using the formalization presented by <cit.>. This method assumes that the spots can be treated as a stellar spectrum but with a lower effective temperature, no surface brightness variation outside the spots, and no plage are present. The effect of these assumptions are a dimming of the star and therefore an increase in the transit depth. <cit.> find for HD 189733b that the change in transit depth due to unocculted spots, Δ(R_p/R_∗) = 2.08×10^-3/2 (R_p/R_∗) between 375–400 nm. Therefore, unocculted spots have minimal influence (assuming our host stars have unocculted spots similar to HD 189733b) on the observed transit depth variations since our final error bars (Table <ref>) are at least 10 times larger than the influence of these spots (e.g. Δ[R_p/R_∗] = 0.00014 for HAT-P-37b). Qualitatively, this result is consistent with the study by <cit.> that find that stellar activity similar to the sun has very little effect on the transit depth measured in near-UV to optical wavelengths. Nonetheless, we highly encourage follow-up observations and host star monitoring of all our targets to assess the effect of stellar activity on the observed transit depth variations. 

Next, we investigate what effect a star-spot crossing in the light curve of HAT-P-37 would have on its calculated transit depth. In the B-band light curve of HAT-P-37b (Figure <ref>) there may be a star-spot crossing at a phase range of 0.004–0.008. However, the detected signal is very close to the scatter in the light curve. If we model the light curve without the possible star-spot crossing we find a (R_p/R_∗) = 0.1278±0.0048, within 1σ of the transit depth of the entire light curve (0.1253±0.0021). <cit.> present a procedure to estimate the effects of unocculted spots on the transit depth. Their procedure can also be used to estimate the effect of star spot crossings on the transit depth, where instead of unocculted spots increasing the transit depth occulted spots should decrease it. <cit.> find that the change in transit depth due to spots, Δ(R_p^2/R_∗^2), is

    Δ(R_p^2/R_∗^2) = ( R_p/R_∗)^2 δT_spot/T_eff,

where R_p/R_∗ is the unperturbed transit depth, δ is the fractional area of star spots, and T_spot is the temperature of the spot. If we set Δ(R_p^2/R_∗^2) = 3100 ppm (the approximate difference between our B-band and the Sloan-i transit depth; ), then we can estimate T_spot and δ. For spot temperatures between 2000–5000 K, we find that δ would be between 20 - 50 %. Typical values of δ for solar-like stars is around several % (e.g. ; ; ), so our estimated δ range is extremely high. Due to both these tests, it seems unlikely that the smaller B-band transit depth of HAT-P-37b is due to an occulted star-spot. 




§ CONCLUSIONS

We observed 11 transiting hot Jupiters (CoRoT-12b, HAT-P-5b, HAT-P-12b, HAT-P-33b, HAT-P-37b, WASP-2b, WASP-24b, WASP-60b, WASP-80b, WASP-103b, XO-3b) from the ground using near-UV and optical filters in order to update their system parameters and constrain their atmospheres. Our observations of CoRoT-12b, HAT-P-37b and WASP-60b are the first follow-up observations of these planets since their discovery and we also obtain the first near-UV light curves of WASP-80b and WASP-103b. We find that HAT-P-5b, HAT-P-12b, WASP-2b, WASP-24b, and WASP-80b exhibit a flat spectrum across the optical wavelengths, suggestive of clouds in their atmospheres. Variation in the transit depths is observed for WASP-103b and XO-3b and may indicate scattering in their atmospheres. Additionally, we observe a smaller B-band transit depth compared to near-IR in HAT-P-37b. Such a variation may be caused by TiO/VO absorption (). We find a larger R-band (which encompasses the Hα line) transit depths in HAT-P-33b and CoRoT-12b and this result may indicate possible atmospheric escape. Follow-up photometry and high-resolution spectroscopy observations are encouraged to confirm all the observed transit depth variations since they are only seen at 2-4.6σ. Our calculated physical parameters agree with previous studies within 1σ with a few exceptions (Tables <ref>–<ref>). For the exoplanets HAT-P-12b, HAT-P-37b, WASP-2b, WASP-24b, WASP-80b, and XO-3b we are able to refine their orbital periods from previous work (Tables <ref>–<ref>). 







 



§ ACKNOWLEDGMENTS


J. Turner, R. Leiter, and R. Johnson were partially supported by the NASA's Planetary Atmospheres program. J. Turner and R. Leiter were also supported by the The Double Hoo Research Grant. J. Turner was also partially funded by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1315231. 

We would like thank Robert T. Zellem for his help with observations. This research has made use of the Exoplanet Orbit Database <cit.>, Exoplanet Data Explorer at exoplanets.org, Exoplanet Transit Database, Extrasolar Planet Transit Finder, NASA's Astrophysics Data System Bibliographic Services, and the International Variable Star Index (VSX) database, operated at AAVSO, Cambridge, Massachusetts, USA. This research has also made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. 
    


    



mnras



