JWST/NIRSpec Detects Warm CO Emission in the Terrestrial-Planet Zone of HD 131488

TL;DR

Using JWST/NIRSpec high-resolution spectroscopy (2.87–5.14 μm) of HD 131488, the study detects warm CO ro-vibrational emission indicating UV fluorescence in gas located within ~10 AU. A two-component model (UV-fluoresced warm gas plus a foreground cold absorber) yields a hot rotational temperature of about $T_{ m rot} oughly 1155 ext{ K}$ and a vibrational temperature near $T_{ m vib} oughly 8800 ext{ K}$ at the inner edge, with a gradient to cooler values out to ~10 AU; the gas is not in LTE. The warm CO mass is constrained to be at least $M_{ m CO} oughly 7.5 imes10^{20}$ g ($ oughly 1.25 imes10^{-7} ext{ M}_{igoplus}$), significantly smaller than the cold outer reservoir detected by ALMA/HST yet detectable via UV fluorescence, implying potential unseen collisional partners such as H$_2$ or H$_2$O. The results demonstrate UV fluorescence as a sensitive probe for tenuous inner-disk gas in debris disks and have implications for gas origin (likely secondary) and for the metallicity evolution of forming planets in the terrestrial zone.

Abstract

We have obtained a high-resolution, JWST NIRSpec $2.87$ -- $5.14$ $μ$m spectrum of the debris disk around HD 131488. We discover CO fundamental emission indicating the presence of warm fluorescent gas within $\sim10$ AU of the star. The large discrepancy in CO's vibrational and rotational temperature indicates that CO is out of thermal equilibrium and is excited with UV fluorescence. Our UV fluorescence model gives a best fit of $1150\,$K with an effective temperature of $450$, $332$, and $125\,$K for the warm CO gas kinetic temperature within $0.5$, $1$, and $10\,$AU to the star and a gas vibrational temperature of $8800\,$K. The newly discovered warm CO gas population likely resides between sub-AU scales and $\sim\,10\,$AU, interior to the cold CO reservoir detected beyond $35\,$AU with HST STIS and ALMA. The discovery of warm, fluorescent gas in a debris disk is the first such detection ever made. The detection of warm CO raises the possibility of unseen molecules (H$_2$O, H$_2$, etc) as collisional partners to excite the warm gas. We estimated a lower mass limit for CO of $1.25\times 10^{-7}\text{M}_{\oplus}$, which is $10^{-5}$ of the cold CO mass detected with ALMA and HST. We demonstrate that UV fluorescence emerges as a promising avenue for detecting tenuous gas at $10^{-7}$ Earth-mass level in debris disks with JWST.

Figures (15)

• Figure 1: HD 131488 Disk and Slit Alignment, showcasing our data is sensitive to the disk region inward of 30 AU to the star. Left: The JWST NIRSpec Slit is overlaid on VLT/SPHERE coronagraphic imaging of scattered light of dust in the HD 131488 disk. The image has been post-processed with Non-negative matrix transformation Xie+22. The alignment between the nearly edge-on disk and the PA of the slit clearly shows that our data is sensitive to the inner region of the disk. Right: Overlay of NIRSpec slit on ALMA $^{12}$CO(2-1) image of HD 131488 Moor+17. The major and minor axes of the disk are assumed to be $1.14^{\prime\prime}$ by $0.7^{\prime\prime}$ according to the disk profile reported in the ALMA CO image Moor+17. For both gas disk and dust disk, our data is sensitive to the disk region inward of a radius of $15$ AU to the star, because the star and inner region of the disk fall in the slit, but are insensitive to the outer region of the disk, because those regions fall outside of the slit.
• Figure 2: A first look at HD 131488 NIRSpec spectrum reveals the presence of multiple vibrational levels of CO ro-vibrational emission, indicating UV fluorescence. Top: HD 131488 NIRSpec spectrum plotted against an identified stellar continuum. Regions affected by cold CO absorption are annotated with a green bracket and text. Features such as artifacts and stellar hydrogen lines are also annotated. Middle: Models of individual vibrational levels are vertically offset for clarity, with $^{13}$CO contributions shaded in opaque blue. Bottom: Zoomed in view for v(9-8) and v(10-9) models. The individual lines or features detected at 5-$\sigma$ after stellar photospheric subtraction are shown in Figure \ref{['fig:sigmaClip']} and detailed in section \ref{['subsec:5sigma']}.
• Figure 3: HD 131488 CO model (blue) compared with NIRSpec data (black).The top panel shows the same best-fit model as in Figure \ref{['fig:COmodel']}, while the remaining panels highlight zoomed-in regions of the spectrum. The short vertical gray lines at 0.176 Jy indicate line or features detected at $5\sigma$ over disk continuum after stellar photosphere subtraction. Middle: Example of a blended line (ROI 1), where each observed feature contains contributions from multiple vibrationally excited transitions. Bottom left: Example of $^{12}$CO versus $^{13}$CO emission (ROI 2), with $^{13}$CO lines shaded in blue and $^{12}$CO vibrational levels shaded according to the scheme in Fig. \ref{['fig:vib-Levels']}. Bottom right: Example of moderate to high rotational lines at highly excited vibrational levels (ROI 3) that constitute features detected at $5~\sigma$ in the observed spectrum.
• Figure 4: Energy Level Schematic Diagram of CO. In this diagram, we consider stimulated absorptions and stimulated+spontaneous emissions driven by UV fluorescence. We choose to ignore IR fluorescence because its transition probability is a few orders of magnitude smaller than that of UV fluorescence. $n_{X}$ and $n_{A}$ stands for the level population of CO at the ground and first excited electronic state $X^{1}\Sigma^{+}$ and $A^{1}\Pi$. $A_{A-X}$ stands for Einstein A coefficients for spontaneous emission (in purple), and $g_{A-X}$ and $g_{A-X}$ stands for Einstein B coefficients for stimulated emission and absorption (in pink). For application to HD 131488, we only consider fundamental transitions which means $k=j+1$.
• Figure 5: Fractional vibrational population excited by the radiation field of an A0 star at 0.1, 1, 4, 10, and 50 AU. $E_{vib}/k$ for the x-axis represents the vibrational constant in units of Kevin. For the CO molecule, the vibrational constant is $3122$ K. So for each vibrational level, $v$, the vibrational energy is $v\,\times\,3122\,$K. As the gas extends outward from $0.1\,\mathrm{AU}$ to $1\,\mathrm{AU}$, the stellar radiation field weakens and becomes more diluted. Over this range, the fitted slope becomes progressively steeper, indicating that rovibrational transitions cool the vibrational levels more efficiently with increasing radial distances. Beyond a few AU ($\sim4$ AU), however, the slope changes only marginally, implying that the effectiveness of this vibrational "cooling’’ saturates and remains nearly constant from $4\,\mathrm{AU}$ out to $50\,\mathrm{AU}$.