Near-Infrared InGaAs Detectors for Background-limited Imaging and Photometry

Originally designed for night-vision equipment, InGaAs detectors are beginning to achieve background-limited performance in broadband imaging from the ground. The lower cost of these detectors can enable multi-band instruments, arrays of small telescopes, and large focal planes that would be uneconomical with high-performance HgCdTe detectors. We developed a camera to operate the FLIR AP1121 sensor using deep thermoelectric cooling and up-the-ramp sampling to minimize noise. We measured a dark current of 163$~e$- s$^{-1}$ pix$^{-1}$, a read noise of 87$~e$- up-the-ramp, and a well depth of 80k$~e$-. Laboratory photometric testing achieved a stability of 230 ppm hr$^{-1/2}$, which would be required for detecting exoplanet transits. InGaAs detectors are also applicable to other branches of near-infrared time-domain astronomy, ranging from brown dwarf weather to gravitational wave follow-up.Comment: Submitted to Proc. SPIE, Astronomical Telescopes + Instrumentation (2014

Precision of a Low-Cost InGaAs Detector for Near Infrared Photometry

We have designed, constructed, and tested an InGaAs near-infrared camera to explore whether low-cost detectors can make small (<1 m) telescopes capable of precise (<1 mmag) infrared photometry of relatively bright targets. The camera is constructed around the 640x512 pixel APS640C sensor built by FLIR Electro-Optical Components. We designed custom analog-to-digital electronics for maximum stability and minimum noise. The InGaAs dark current halves with every 7 deg C of cooling, and we reduce it to 840 e-/s/pixel (with a pixel-to-pixel variation of +/-200 e-/s/pixel) by cooling the array to -20 deg C. Beyond this point, glow from the readout dominates. The single-sample read noise of 149 e- is reduced to 54 e- through up-the-ramp sampling. Laboratory testing with a star field generated by a lenslet array shows that 2-star differential photometry is possible to a precision of 631 +/-205 ppm (0.68 mmag) hr^-0.5 at a flux of 2.4E4 e-/s. Employing three comparison stars and de-correlating reference signals further improves the precision to 483 +/-161 ppm (0.52 mmag) hr^-0.5. Photometric observations of HD80606 and HD80607 (J=7.7 and 7.8) in the Y band shows that differential photometry to a precision of 415 ppm (0.45 mmag) hr^-0.5 is achieved with an effective telescope aperture of 0.25 m. Next-generation InGaAs detectors should indeed enable Poisson-limited photometry of brighter dwarfs with particular advantage for late-M and L types. In addition, one might acquire near-infrared photometry simultaneously with optical photometry or radial velocity measurements to maximize the return of exoplanet searches with small telescopes.Comment: Accepted to PAS

Kepler Transit Depths Contaminated by a Phantom Star

We present ground-based observations from the Discovery Channel Telescope (DCT) of three transits of Kepler-445c---a supposed super-Earth exoplanet with properties resembling GJ 1214b---and demonstrate that the transit depth is approximately 50 percent shallower than the depth previously inferred from Kepler Spacecraft data. The resulting decrease in planetary radius significantly alters the interpretation of the exoplanet's bulk composition. Despite the faintness of the M4 dwarf host star, our ground-based photometry clearly recovers each transit and achieves repeatable 1-sigma precision of approximately 0.2 percent (2 millimags). The transit parameters estimated from the DCT data are discrepant with those inferred from the Kepler data to at least 17-sigma confidence. This inconsistency is due to a subtle miscalculation of the stellar crowding metric during the Kepler pre-search data conditioning (PDC). The crowding metric, or CROWDSAP, is contaminated by a non-existent "phantom star" originating in the USNO-B1 catalog and inherited by the Kepler Input Catalog (KIC). Phantom stars in the KIC are likely rare, but they have the potential to affect statistical studies of Kepler targets that use the PDC transit depths for a large number of exoplanets where individual follow-up observation of each is not possible. The miscalculation of Kepler-445c's transit depth emphasizes the importance of stellar crowding in the Kepler data, and provides a cautionary tale for the analysis of data from the Transiting Exoplanet Survey Satellite (TESS), which will have even larger pixels than Kepler.Comment: 11 pages, 10 figures, 5 tables. Accepted for publication in AJ. Transit light curves will be available from AJ as Db

Broadband Transmission Spectroscopy of the super-Earth GJ 1214b suggests a Low Mean Molecular Weight Atmosphere

We used WIRCam on CFHT to observe four transits of the super-Earth GJ 1214b in the near-infrared. For each transit we observed in two bands nearly-simultaneously by rapidly switching the WIRCam filter wheel back and forth for the duration of the observations. By combining all our J-band (~1.25 microns) observations we find a transit depth in this band of 1.338\pm0.013% - a value consistent with the optical transit depth reported by Charbonneau and collaborators. However, our best-fit combined Ks-band (~2.15 microns) transit depth is deeper: 1.438\pm0.019%. Formally our Ks-band transits are deeper than the J-band transits observed simultaneously by a factor of 1.072\pm0.018 - a 4-sigma discrepancy. The most straightforward explanation for our deeper Ks-band depth is a spectral absorption feature from the limb of the atmosphere of the planet; for the spectral absorption feature to be this prominent the atmosphere of GJ 1214b must have a large scale height and a low mean molecular weight. That is, it would have to be hydrogen/helium dominated and this planet would be better described as a mini-Neptune. However, recently published observations from 0.78 - 1.0 microns, by Bean and collaborators, show a lack of spectral features and transit depths consistent with those obtained by Charbonneau and collaborators. The most likely atmospheric composition for GJ 1214b that arises from combining all these observations is less clear; if the atmosphere of GJ 1214b is hydrogen/helium dominated then it must have either a haze layer that is obscuring transit depth differences at shorter wavelengths, or significantly different spectral features than current models predict. Our observations disfavour a water-world composition, but such a composition will remain a possibility until observations reconfirm our deeper Ks-band transit depth or detect features at other wavelengths. [Abridged]Comment: ApJ accepted. 12 pages, 6 figures, in EmulateApJ forma

Near-Infrared Thermal Emission from the Hot Jupiter TrES-2b: Ground-Based Detection of the Secondary Eclipse

We present near-infrared Ks-band photometry bracketing the secondary eclipse of the hot Jupiter TrES-2b using the Wide-field Infrared Camera on the Canada-France-Hawaii Telescope. We detect its thermal emission with an eclipse depth of 0.062 +/- 0.012% (5-sigma). Our best-fit secondary eclipse is consistent with a circular orbit (a 3-sigma upper limit on the eccentricity, e, and argument or periastron, omega, of |ecos(omega)| < 0.0090), in agreement with mid-infrared detections of the secondary eclipse of this planet. A secondary eclipse of this depth corresponds to a day-side Ks-band brightness temperature of TB = 1636 +/- 88 K. Our thermal emission measurement when combined with the thermal emission measurements using Spitzer/IRAC from O'Donovan and collaborators suggest that this planet exhibits relatively efficient day to night-side redistribution of heat and a near isothermal dayside atmospheric temperature structure, with a spectrum that is well approximated by a blackbody. It is unclear if the atmosphere of TrES-2b requires a temperature inversion; if it does it is likely due to chemical species other than TiO/VO as the atmosphere of TrES-2b is too cool to allow TiO/VO to remain in gaseous form. Our secondary eclipse has the smallest depth of any detected from the ground at around 2 micron to date.Comment: ApJ accepted, 8 pages, 9 figures, in emulateapj format

Multiwavelength transit observations of the candidate disintegrating planetesimals orbiting WD 1145+017

We present multiwavelength, ground-based follow-up photometry of the white dwarf WD 1145+017, which has recently been suggested to be orbited by up to six or more short-period, low-mass, disintegrating planetesimals. We detect nine significant dips in flux of between 10% and 30% of the stellar flux in our ~32 hr of photometry, suggesting that WD 1145+017 is indeed being orbited by multiple, short-period objects. Through fits to the asymmetric transits that we observe, we confirm that the transit egress is usually longer than the ingress, and that the transit duration is longer than expected for a solid body at these short periods, all suggesting that these objects have cometary tails streaming behind them. The precise orbital periods of the planetesimals are unclear, but at least one object, and likely more, have orbital periods of ~4.5 hr. We are otherwise unable to confirm the specific periods that have been reported, bringing into question the long-term stability of these periods. Our high-precision photometry also displays low-amplitude variations, suggesting that dusty material is consistently passing in front of the white dwarf, either from discarded material from these disintegrating planetesimals or from the detected dusty debris disk. We compare the transit depths in the V- and R-bands of our multiwavelength photometry, and find no significant difference; therefore, for likely compositions, the radius of single-size particles in the cometary tails streaming behind the planetesimals must be ~0.15 μm or larger, or ~0.06 μm or smaller, with 2σ confidence

Near-Infrared Thermal Emission from TrES-3b: A Ks-band detection and an H-band upper limit on the depth of the secondary eclipse

We present H and Ks-band photometry bracketing the secondary eclipse of the hot Jupiter TrES-3b using the Wide-field Infrared Camera on the Canada-France-Hawaii Telescope. We detect the secondary eclipse of TrES-3b with a depth of 0.133+/-0.017% in Ks-band (8-sigma) - a result in sharp contrast to the eclipse depth reported by de Mooij & Snellen. We do not detect its thermal emission in H-band, but place a 3-sigma limit on the depth of the secondary eclipse in this band of 0.051%. A secondary eclipse of this depth in Ks requires very efficient day-to-nightside redistribution of heat and nearly isotropic reradiation, conclusion that is in agreement with longer wavelength, mid-infrared Spitzer observations. Our 3-sigma upper-limit on the depth of our H-band secondary eclipse also argues for very efficient redistribution of heat and suggests that the atmospheric layer probed by these observations may be well homogenized. However, our H-band upper limit is so constraining that it suggests the possibility of a temperature inversion at depth, or an absorbing molecule, such as methane, that further depresses the emitted flux at this wavelength. The combination of our near-infrared measurements and those obtained with Spitzer suggest that TrES-3b displays a near isothermal dayside atmospheric temperature structure, whose spectrum is well approximated by a blackbody. We emphasize that our strict H-band limit is in stark disagreement with the best-fit atmospheric model that results from longer wavelength observations only, thus highlighting the importance of near-infrared observations at multiple wavelengths in addition to those returned by Spitzer in the mid-infrared to facilitate a comprehensive understanding of the energy budgets of transiting exoplanets.Comment: ApJ accepted, 8 pages, 7 figures, in EmulateApJ forma