Clouds with a silicate lining: Using JWST spectra to probe atmospheric diversity in young AB Dor L dwarfs

Abstract

We present the first full JWST NIRSpec Prism and MIRI LRS 0.6 - 14 $μ$m (R ~ 100) spectra and analysis of five ~ 133 Myr L dwarf members of the AB Doradus moving group and one probable $\sim 500$ Myr T dwarf of the Oceanus moving group with known inclination angles between ~ $23 - 90^{\circ}$: W0047+68, 2M0355+11, 2M0642+41, W1741-46, 2M2206-42, and 2M2244+20. We construct near-complete spectral energy distributions of each of our objects to measure their bolometric luminosities, and estimate their fundamental parameters ($T_{\text{eff}}$, radius, $M$ and $\log g$). We use cross-sections of relevant gases to identify the species that are present in each atmosphere. Of particular interest is the silicate absorption feature at 8 - 11 $μ$m, which provides insight into the complex cloud structure of brown dwarfs. We examine this silicate absorption feature in detail and also test whether there exists a latitudinal dependence in the silicate absorption feature within a coeval sample of brown dwarfs. Various molecular absorption bands are visible in our spectra, including H$_2$O, CH$_4$, CO and CO$_2$. The shape of the silicate absorption feature varies within our sample, and we find that 4/5 of our L type objects agree with previously observed trends stating that objects viewed equator-on have deeper silicate absorption. We highlight W1741-46 as an outlier in our sample with an unusually strong silicate absorption given its near pole-on orientation. We also present a tentative correlation between the wavelength of peak silicate absorption and inclination, which may suggest variations in cloud chemical composition or physical properties. We find an unexpected spectral diversity within our sample, which motivates future studies on these objects through atmospheric retrievals, which will determine the silicate cloud composition and reveal whether there exists a trend with inclination.

Figures (14)

• Figure 1: Colour-magnitude diagram indicating locations of our targets (coloured stars) at the L-T transition. The location of VHS 1256 b has also been included as a cyan circle. Planets in the HR 8799 system are also indicated as black circles. PSO J318 is indicated as a pink circle. The coral points are M dwarfs, dark red points are L dwarfs, and blue points are T dwarfs. Photometric data for each point was obtained from the Ultracool Sheet UltraCoolSheet.
• Figure 2: Distance-calibrated spectra for each object (top panel). Shaded regions show key absorption bands for CH$_4$ , CO, CO$_2$ , NH$_3$ and H$_2$O . The dotted line shows the offset level for each spectrum. The spectra are ordered by decreasing effective temperature from top to bottom. The cross sections for each species, computed at $T = 1300$ K and $P = 1$ bar, are included in the bottom panels.
• Figure 3: JWST/NIRSpec Prism spectra of our sample (coloured) compared to their respective best fitting spectral standard (black) after dereddening (Bickle et al. in prep.).
• Figure 4: JWST/NIRSpec Prism NIR spectrum of 2M0642$+$41 (black), dereddened and compared to L/T transition low-gravity spectral standards. The T1$\gamma$ spectral standard is the best fit to 2M0642$+$41 . CH$_4$ absorption regions are shaded in orange.
• Figure 5: Visualisation of the fundamental parameters of the sample analysed in Sanghi2023 (grey circles) and our sample (coloured outlined stars). $T_{\text{eff}}$ is plotted against IR spectral type. The fill colour is indicative of the mass of the object. Random noise of 0.3 spectral types was added along the x-axis to minimise overlapping points in visualisation. The fundamental parameters were calculated using evolutionary models. The polynomial relations for the old field dwarfs and young moving group members (blue and red lines, respectively) from Sanghi2023 are overplotted.