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The ExoGRAVITY survey: A K-band spectral library of giant exoplanet and brown dwarf companions

J. Kammerer, T. O. Winterhalder, S. Lacour, T. Stolker, G. -D. Marleau, W. O. Balmer, A. F. Moore, L. Piscarreta, C. Toci, A. Mérand, M. Nowak, E. L. Rickman, L. Pueyo, N. Pourré, E. Nasedkin, J. J. Wang, G. Bourdarot, F. Eisenhauer, Th. Henning, R. Garcia Lopez, E. F. van Dishoeck, T. Forveille, J. D. Monnier, R. Abuter, A. Amorim, M. Benisty, J. -P. Berger, H. Beust, S. Blunt, A. Boccaletti, M. Bonnefoy, H. Bonnet, M. S. Bordoni, W. Brandner, F. Cantalloube, P. Caselli, W. Ceva, B. Charnay, G. Chauvin, A. Chavez, A. Chomez, E. Choquet, V. Christiaens, Y. Clénet, V. Coudé du Foresto, A. Cridland, R. Davies, R. Dembet, J. Dexter, A. Drescher, G. Duvert, A. Eckart, C. Fontanive, N. M. Förster Schreiber, P. Garcia, E. Gendron, R. Genzel, S. Gillessen, J. H. Girard, S. Grant, J. Hagelberg, X. Haubois, G. Heißel, S. Hinkley, S. Hippler, M. Houllé, Z. Hubert, L. Jocou, M. Keppler, P. Kervella, L. Kreidberg, N. T. Kurtovic, A. -M. Lagrange, V. Lapeyrère, J. -B. Le Bouquin, D. Lutz, A. -L. Maire, F. Mang, E. C. Matthews, P. Mollière, C. Mordasini, D. Mouillet, T. Ott, G. P. P. L. Otten, C. Paladini, T. Paumard, K. Perraut, G. Perrin, O. Pfuhl, D. C. Ribeiro, Z. Rustamkulov, D. Ségransan, J. Shangguan, T. Shimizu, M. Samland, D. Sing, J. Stadler, O. Straub, C. Straubmeier, E. Sturm, L. J. Tacconi, S. Udry, A. Vigan, F. Vincent, S. D. von Fellenberg, F. Widmann, J. Woillez, S. Yazici, the GRAVITY Collaboration

Abstract

Direct observations of exoplanet and brown dwarf companions with near-infrared interferometry, first enabled by the dual-field mode of VLTI/GRAVITY, provide unique measurements of the objects' orbital motions and atmospheric compositions. Here, we compile a homogeneous library of all exoplanet and brown dwarf K-band spectra observed by GRAVITY thus far. We re-reduced all the available GRAVITY dual-field high-contrast data and, where companions are detected, extract their ~2.0-2.4 $μ$m K-band contrast spectra. We then derived stellar model atmospheres for all employed flux references, which we used to convert the companion contrast into companion flux spectra. Solely from the resulting GRAVITY spectra, we extracted spectral types, spectral indices, and bulk physical properties for all companions. Finally, and with the help of age constraints from the literature, we also derived isochronal masses for most companions using evolutionary models. The resulting library contains R ~ 500 GRAVITY spectra of 39 substellar companions from late M to late T spectral types, including the entire L-T transition. Throughout this transition, a shift from CO-dominated late M- and L-type dwarfs to CH4-dominated T-type dwarfs can be observed in the K-band. The GRAVITY spectra alone constrain the objects' bolometric luminosity to typically within $\pm$0.15 dex. The derived isochronal masses agree with dynamical masses from the literature where available, except for HD 4113 c for which we confirm its previously reported potential underluminosity. Medium-resolution spectroscopy of substellar companions with GRAVITY provides insight into the carbon chemistry and the cloudiness of these objects' atmospheres. It also constrains these objects' bolometric luminosities which can yield measurements of their formation entropy if combined with dynamical masses, for instance from Gaia and GRAVITY astrometry.

The ExoGRAVITY survey: A K-band spectral library of giant exoplanet and brown dwarf companions

Abstract

Direct observations of exoplanet and brown dwarf companions with near-infrared interferometry, first enabled by the dual-field mode of VLTI/GRAVITY, provide unique measurements of the objects' orbital motions and atmospheric compositions. Here, we compile a homogeneous library of all exoplanet and brown dwarf K-band spectra observed by GRAVITY thus far. We re-reduced all the available GRAVITY dual-field high-contrast data and, where companions are detected, extract their ~2.0-2.4 m K-band contrast spectra. We then derived stellar model atmospheres for all employed flux references, which we used to convert the companion contrast into companion flux spectra. Solely from the resulting GRAVITY spectra, we extracted spectral types, spectral indices, and bulk physical properties for all companions. Finally, and with the help of age constraints from the literature, we also derived isochronal masses for most companions using evolutionary models. The resulting library contains R ~ 500 GRAVITY spectra of 39 substellar companions from late M to late T spectral types, including the entire L-T transition. Throughout this transition, a shift from CO-dominated late M- and L-type dwarfs to CH4-dominated T-type dwarfs can be observed in the K-band. The GRAVITY spectra alone constrain the objects' bolometric luminosity to typically within 0.15 dex. The derived isochronal masses agree with dynamical masses from the literature where available, except for HD 4113 c for which we confirm its previously reported potential underluminosity. Medium-resolution spectroscopy of substellar companions with GRAVITY provides insight into the carbon chemistry and the cloudiness of these objects' atmospheres. It also constrains these objects' bolometric luminosities which can yield measurements of their formation entropy if combined with dynamical masses, for instance from Gaia and GRAVITY astrometry.

Paper Structure

This paper contains 22 sections, 8 equations, 11 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Observations and data reduction
  3. GRAVITY observations
  4. GRAVITY data reduction
  5. Flux calibration using stellar model atmospheres
  6. Combining spectra from multiple epochs
  7. Systematics in the GRAVITY spectra
  8. Description of data format
  9. Results
  10. Spectral types
  11. GRAVITY K-band spectra
  12. Spectral indices
  13. Discussion
  14. Atmospheric characterization
  15. Comparison with evolutionary models
  16. ...and 7 more sections

Figures (11)

  • Figure 1: Combination of the GRAVITY epochs of HD 206505 b after scaling to bring them into mutual agreement. Top panel: GRAVITY K-band flux spectra for each individual epoch. The $3\sigma$ flux uncertainties are shown as shaded regions. The legend reveals that for this object both observations were done in dual-field on-axis mode, with the companion located at an angular separation of $425$ mas from the host star. Bottom panel: Scaled GRAVITY flux spectra for each individual epoch and the resulting combined spectrum (black line). The legend reveals the scale factors applied to each epoch (values in brackets).
  • Figure 2: Distribution of scale factors applied to those GRAVITY companion spectra that were flux-calibrated using the host star. The spectra were scaled to bring individual epochs of the same object into agreement. The median scale factor is shown by the solid black line and the 16th and 84th percentiles are shown by the dashed black lines.
  • Figure 3: GRAVITY K-band spectra at medium spectral resolution ($R \sim 500$) of the 22 substellar companions with a well-constrained spectral type, sorted from M-type (top) to T-type (bottom). The spectra are scaled with respect to each other by the ratio of the square root of their S/N to their mean flux, and they are offset along the y-axis so that their mean value is located at the height of their corresponding tick mark for better readability. The best fitting spectral type templates from the SpeX Prism Libraries are shown in gray with their corresponding K1-K2 extinction $E_K$ in units of magnitudes shown on the left. The S/N of each spectrum is shown on the right.
  • Figure 4: K1-K2 color magnitude diagram for 38 of the 39 objects in the ExoGRAVITY Spectral Library. AMES-Cond and AMES-Dusty evolutionary tracks at an age of 20 Myr (solid lines) and 100 Myr (dashed lines) are plotted in the background and mid-M-type to late T-type objects from the SpeX Prism Libraries are shown for reference. HD 167665 b is missing from the plot because its methane absorption is strong, thus yielding only an upper flux limit in K2.
  • Figure 5: Methane- (CH${}_4$), carbon-monoxide- (CO), and water- (WK) sensitive spectral indices for all objects in the ExoGRAVITY Spectral Library. For reference, young dwarfs ($<10$ Myr) and field dwarfs from the literature are shown in the background. For the ExoGRAVITY sample, objects are considered young if they have an estimated age of $<50$ Myr or if they are members of the Argus moving group.
  • ...and 6 more figures