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Spectroscopic Observations of HD 209458 b

Lewis Kotredes

1 Introduction

According to the California and Carnegie Search website [1] there are currently 110 known orbiting around other than our . With a very few notable exceptions, all of these planets were detected by surveys of local stars, itself an interesting spectroscopic technique. However, only two are known to orbit in such a way that the planet passes directly between the host and the Earth. These transiting extrasolar planets are HD 209458 b and OGLE TR-56 b. Transiting planets are inter- esting and unique for a number of reasons. The can be found directly, given radial velocity measurements of the host stars, and the radii of both star and planet can also be determined. Another valuable use of transiting planets is the spectroscopic examination of these planets through the gen- eral technique known as transmission . Unfortunately, the host star OGLE TR-56 is relatively distant, and the detection of transits around this star is sufficiently recent that work using these techniques is at best preliminary; for this reason this paper focuses exclusively on HD 209458 b.

2 Spectroscopic Techniques

The general intent of transmission spectroscopy is rather intuitive. Because of the proximity of known extrasolar planets to their stars, and the huge contrast ratio between the stars and their planets, it is not currently possible to detect the planets directly. This means that obtaining spectra of the planets is futile. However, when the planet passes directly in front of the star, some starlight is passing through the planet’s before being observed in the . Any species which create spectroscopic lines in

1 this atmosphere, then, can be observed. This can be done in several ways; through narrow band filters and through direct spectroscopy. The narrow-band filter technique is simple, but does not tell us everything we would like to know about the observed spectral features. This technique simply observes transits using a narrow-band filter centered on a spectral line and photometrically observes the light curve of a . This light curve has a depth and duration determined by the details of the size of the planet and the optically thick regions of the atmosphere. The light curve is compared to those taken by wider filters, which average out spectral features and are assumed to be consistent with an opaque planet, without atmospheric contributions. [2] The technique of using direct spectroscopy is somewhat more challenging. Spectra of the star/planet system are taken both when the system is under- going a transit and when it is not. Then, by subtracting the stellar spectrum from the transit spectrum, the spectrum of the planet itself is determined. This technique requires exacting calibration, especially on features which may be found in both planet and star, but interesting things can be learned. For example, this technique theoretically allows us to learn about the line profile of the species in the planet, which allows for determination of pressure and in the planetary atmosphere, data of primary importance to modellers trying to distinguish between a large number of possible models for extrasolar planets and their formation. [3] A similar, but even more difficult, technique is also sometimes used to directly detect spectral features from the surface of the planet. In this tech- nique, known as occultation spectroscopy, instead of looking for spectral fea- tures during the primary eclipse, we examine for reflected spectral features by differencing with the secondary eclipse. This technique is very difficult due to the small contrast ratios involved, and is more often used for the determination of upper bounds than direct measurements at present. [4]

3 HD 209458 b

The planet known as HD 209458 b (sometimes called Osiris) was initially discovered using radial velocity methods. In 2000 a team of observers de- tected the distinct photometric signature of this planet transiting its host star, and shortly thereafter searches for spectral features using transmission spectroscopy were begun [5]. In 2002, a team using STIS on Hubble de-

2 tected Na D lines at 5893 ˚A using the narrow-band technique. The bandpass used was 12 ˚A wide, and the change in the measured transit depth was (2.32  0.57) × 10−4. With an assumed planetary temperature used to de- termine a scale height and assuming a cloudless model, this detection was a factor of 3 smaller than predicted, data which have been used to some effect in late model production. [2] [6] Another team, again using STIS but this time using the G140M grating and MAMA detector, searched for the Lyman α line of by com- paring spectra before and during transit on three different transits. These observations revealed that in this line the transit depth, instead of the 1.5% transit depth measured in wide filters, was actually 15%. This indicates that the atmosphere in hydrogen has a radius of greater than 3 planetary radii. In fact, since the Roche lobe of this planet is only 2.7 planetary radii, this suggests that the planet is losing at a considerable rate, a fact which has caused the leader of the team that found this effect to suggest the name Osiris (after the Egyptian god of death) for this planet. [3] In 2004, the group responsible for detecting atomic hydrogen in the at- mosphere of HD 209458 b also reported detections of O I and C II in the upper atmosphere of the planet. The low spectral resolution of the grating was insufficient to resolve emission lines or to separate the blends mentioned below. The line detected had a transit depth of 10  3.5%, and is a blend of a ground state line at 1302.2 ˚A with two excited state lines at 1304.9 ˚A and 1306.0 ˚A (the energy levels of these two states are 158 cm−1 and 227 cm−1). However, this ground state is strongly absorbed by the interstellar medium and thus will not be changed by differencing the spectra, suggesting that the absorption lines in the spectrum are actually due to the excited states. Since these states are collisionally excited, this can be used to put limits on the of atomic hydrogen in the upper atmosphere. Similarly, the line has a depth of 6  3% and is a blend between the ground state line at 1334.5 ˚A and an excited state at 1335.7 ˚A (excited state has an energy of 63.42 cm−1). The interstellar absorption of the C II line is not as strong as that of neutral oxygen, so this does not provide a similarly strong constraint on hydrogen density, although the results for oxygen suggest that there should be collisionally excited carbon present. [7] The occultation spectroscopy technique has also been attempted on HD 209458 b. At least one example was an attempt to measure spectral features near 2.2 microns in this way. This technique has been used to provide limits as low as 3 × 10−4 of the stellar flux, which is sufficient to rule out some

3 models of the planetary atmosphere. A primary target for detection in this manner is the CO molecule, but a brief search did not report any definite detections using this technique. [4]

4 Conclusions

So far, a number of techniques have been applied to examine the atmosphere of HD 209458 b. Using these techniques, spectral features associated with atomic , hydrogen, oxygen and carbon have all been successfully de- tected. As the number of observations and techniques applied to the search increases, more detections are expected. An increase in the number of tran- siting planets will also increase the usefulness of these techniques, as we examine more planets outside of our solar system.

References

[1] http://exoplanets.org

[2] Charbonneau, D., 2003, ASP Conf. Ser. ”Scientific Frontiers on Research of Extrasolar Planets”

[3] Vidal-Madjar, A. et al., 2003, Nature, 422, 143

[4] Richardson, L. J. et al., 2004, ASP Conf. Ser. ”Extrasolar Planets, Today and Tomorrow”

[5] Charbonneau, D., et al., 2000, ApJ, 529, L45

[6] Seager, S., 2003, ASP Conf. Ser. ”Scientific Frontiers on Research of Extrasolar Planets”

[7] Vidal-Madjar, A. et al., 2004, astro-ph/0401457

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