Source-linked AI summary
Metals in the Exosphere of the Highly Irradiated Planet WASP-12b
L. Fossati, C. A. Haswell, C. S. Froning, L. Hebb, S. Holmes, U. Kolb, C. Helling, A. Carter, P. Wheatley, A. C. Cameron, B. Loeillet, D. Pollacco, R. Street, H. C. Stempels, E. Simpson, S. Udry, Y. C. Joshi, R. G. West, I. Skillen, D. Wilson
TL;DR
Stellar Ly α variability complicates observations of hydrogen, motivating near-UV observations of WASP-12 with the Cosmic Origins Spectrograph. Three independent analyses indicate metal absorption from an extended exosphere, with effective radii implying it overfills the Roche lobe, although the observed NUVA early ingress lacks a detailed explanation.
Problem
Temporal and spatial variability of stellar Ly α emission complicates observing hydrogen.
Method
The study observes WASP-12 in the near-UV using the Cosmic Origins Spectrograph to obtain better signal to noise.
Results
Three independent analyses indicate absorption in resonance lines of metals from an extended atmosphere surrounding WASP-12b, with effective planet radii of 2.69±0.24 RJ, 2.18±0.18 RJ, and 2.66±0.22 RJ.
Takeaways & Limitations
Taken as a whole, the results constitute compelling evidence that WASP-12b is surrounded by an exosphere over-filling the planet’s Roche lobe.
Takeaways & Limitations
The study does not have a detailed explanation for the observed early ingress in NUVA.
Abstract
from arXiv · showhide
We present near-UV transmission spectroscopy of the highly irradiated transiting exoplanet WASP-12b, obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope. The spectra cover three distinct wavelength ranges: NUVA (2539-2580 Å), NUVB (2655-2696 Å), and NUVC (2770-2811 Å). Three independent methods all reveal enhanced transit depths attributable to absorption by resonance lines of metals in the exosphere of WASP-12b. Light curves of total counts in the NUVA and NUVC wavelength ranges show a detection at a 2.5σ level. We detect extra absorption in the Mg II λλ2800 resonance line cores at the 2.8σ level. The NUVA, NUVB, and NUVC light curves imply effective radii of 2.69+/-0.24 RJ, 2.18+/-0.18 RJ, and 2.66+/-0.22 RJ respectively, suggesting the planet is surrounded by an absorbing cloud which overfills the Roche lobe. We detect enhanced transit depths at the wavelengths of resonance lines of neutral sodium, tin, and manganese, and at singly ionized ytterbium, scandium, manganese, aluminum, vanadium, and magnesium. We also find the statistically expected number of anomalous transit depths at wavelengths not associated with any known resonance line. Our data are limited by photon noise, but taken as a whole the results are strong evidence for an extended absorbing exosphere surrounding the planet. The NUVA data exhibit an early ingress, contrary to model expectations; we speculate this could be due to the presence of a disk of previously stripped material.
1. Introduction
WASP-12b is an attractive target for studying extended planetary atmospheres because it is highly irradiated and closely orbiting its host star. Near-UV resonance lines offer probes of atomic and ionic species while avoiding the variability affecting stellar Ly α observations.
- Extended clouds around similarly observed planets have been associated with enhanced effective radii comparable to or larger than the Roche lobe.Prior interpretations attributed this to hydrodynamic atmospheric blow-off, while energetic neutral atoms were proposed as an alternative.
- WASP-12b is one of the hottest and most irradiated transiting exoplanets and orbits extremely close to a late F-type host star.
- WASP-12b could help distinguish between suggested causes of the extended-atmosphere phenomenon observed in other transiting exoplanets.
- Temporal and spatial variability of stellar Ly α emission is a highly undesirable complicating factor for far-UV observations.
- Near-UV observations provide many resonance lines, including strong Mg II lines, and were chosen to obtain better signal to noise than far-UV Ly α observations.UV resonance lines from the ground state are sensitive probes of atomic and ionic species.
2. Observations and data reduction
The study obtained time-tagged COS near-UV spectra of WASP-12b across three wavelength ranges and analyzed background-subtracted count-rate time series. The data are photon-noise limited and contain strong blended stellar absorption, especially around the Mg II doublet.
- The COS G285M observations covered NUVA: 2539–2580 Å, NUVB: 2655–2696 Å, and NUVC: 2770–2811 Å at R ∼20 000.The spectra were obtained in TIME-TAG mode over five consecutive HST orbits.
- Count rates were analyzed after background subtraction rather than using flux-calibrated spectra.
- Photon statistics dominated the uncertainties because the NUV channel had a high-quality flat field and relatively low background.
- The wavelength-summed count rates were roughly 10 count s−1, 28 count s−1, and 13 count s−1 for NUVA, NUVB, and NUVC, respectively.
- The NUVB spectrum achieved an SNR of ∼10 per pixel for each 3000 sec exposure.
- All three regions contain many blended photospheric absorption lines and no unabsorbed stellar continuum; NUVC is strongly absorbed by the Mg II doublet at 2795.5 Å and 2802.7 Å.NUVB lies closest to the continuum among the three regions.
3. Detection of a wavelength dependent planet transit
Three complementary analyses find wavelength-dependent transit signals in WASP-12b’s near-UV spectrum, including deeper NUVA and NUVC transits and absorption associated with Mg II and other resonance lines.
- 3.1. The Mg II lines: Three methods test whether transit depth varies with wavelength by comparing spectral bands, light curves, and ratio spectra.The Mg II analysis uses blue, center, and red regions with narrow, medium, and wide center bands.
- 3.1. The Mg II lines: 2.8σ deeper transit absorption is detected in the wide Mg II-centered band, while the medium-band signal is 1.1σ and the narrow-band signal is not detected.The narrow-band non-detection is attributed to low central-band signal and absorption in the wider comparison bands.
- 3.2. The transit light curve: The NUVA data suggest an early ingress, with the second exposure approximately 2σ below the out-of-transit level.Splitting that exposure into three sub-exposures also suggests early ingress in NUVA.
4. Discussion
Three independent analyses support metal absorption in an extended WASP-12b exosphere, whose inferred extent overfills the planet’s Roche lobe. The observations also reveal species-dependent transit behavior and an unexplained early ingress in NUVA.
- Metal absorption: Three independent analyses suggest absorption in resonance lines of metals from WASP-12b’s extended atmosphere.Detected species include neutral sodium, tin, and manganese, plus singly ionized ytterbium, scandium, manganese, aluminum, vanadium, and magnesium.
- Extent of the absorbing cloud: 2.69±0.24 RJ, 2.18±0.18 RJ, and 2.66±0.22 RJ are the effective radii implied by NUVA, NUVB, and NUVC, respectively.WASP-12b’s optical radius is 1.79 ± 0.09 RJ, while its mean Roche lobe radius is 2.36 RJ.
- Metal absorption: Enhanced transit depths occur at resonance-line wavelengths, while the number of anomalous depths away from known resonance lines is statistically expected.The authors also report enhanced depths in the Mg II doublet and other metal or ion resonance lines.
- Extent of the absorbing cloud: The combined results constitute compelling evidence that WASP-12b is surrounded by an exosphere that overfills its Roche lobe.The exosphere is likely composed of multiple elements and ions and is expected to be hydrogen rich.
- Transit-shape interpretation: NUVA shows an early ingress, although the authors have no detailed explanation and note that diffuse-cloud models instead suggest delayed egress.They speculate that previously stripped material could form a diffuse ring or torus around the star and compress in front of the planet.
- Transit-shape interpretation: Different transit shapes across wavelength regions may reflect element- or ion-dependent behavior under strong radiation pressure.The NUVA excess may arise from cumulative absorption by many weak lines, whereas NUVC absorption is associated with the Mg II doublet; NUVB is least deviant from the optical transit.