Spitzer + HST parallaxes of 13 late T and Y dwarfs

Abstract

We present astrometric measurements for 13 cold brown dwarfs in the solar neighborhood (d < 20pc). By combining archival Spitzer data with our own Hubble Space Telescope (HST) observations, we achieve parallax uncertainties typically around 10%. Using Spitzer and HST photometry we compare our targets with other known late T and Y dwarfs in the Solar neighborhood, confirming that there is large intrinsic scatter in the near- and mid-infrared absolute magnitudes and colors of this population, further highlighting the diversity observed spectroscopically by several James Webb Space Telescope (JWST) programs. This scatter makes photometric distance estimates highly unreliable and, therefore, makes astrometric parallax measurements fundamental for a meaningful characterization of even the nearest cold brown dwarfs.

Figures (6)

• Figure 1: Comparison of the uncertainties on the best-fit parameters of our astrometric model obtained when using the full unWISE + Spitzer + HST dataset vs. using only the Spitzer + HST data. Uncertainties on the reference coordinates (top panel) are greatly improved when omitting unWISE data, while those on proper motion (middle panel) and parallax (bottom panel) are mostly unchanged. Objects with more uncertain proper motions (i.e. $\sigma_\mu > 15$ mas yr$^{-1}$) or parallaxes (i.e. $\sigma_\varpi > 10$ mas) show the largest deviations from the identity line (dashed line).
• Figure 2: Astrometric fit for CWISEP J023842.60$-$133210.7. (Upper left) A square patch of sky showing the measured coordinates and their uncertainties at each epoch (black points with error bars). Points with small error bars are the Spitzer and HST measurements, labeled for clarity; those with larger error bars are the unWISE measurements. The blue curve shows the best fit from the vantage point of Spitzer. The orange curve shows the same fit as seen from the vantage point of WISE/NEOWISE and HST (i.e. the Earth). Red lines connect each observation to its corresponding point along the best-fit curve. (Upper right) A square patch of sky centered at the mean position of the target. The green ellipse is the parallactic fit. For clarity, only the Spitzer and HST measurements are shown, in blue and orange respectively. Though not shown, the unWISE measurements are included in the fit. Again, red lines connect the time of the observation with its prediction. In the background is the ecliptic coordinate grid, with lines of constant $\beta$ shown in solid pale purple and lines of constant $\lambda$ shown in dashed pale purple. (Lower left) The change in R.A. and decl. as a function of time with the proper motion component removed. The parallactic fit from the vantage point of Spitzer is shown in blue, and from the vantage point of HST is shown in orange. Again, only the Spitzer and HST measurements are shown, in blue and orange respectively. (Lower right) The R.A. and decl. residuals from the fit as a function of time. As with the lower left panel, only the Spitzer and HST data are shown, in blue and orange respectively.
• Figure 3: Same as Figure \ref{['fig:plx_fit_unwise']}, but without using unWISE data.
• Figure 4: Top: Color-magnitude diagram for our sample compared to known nearby L, T, and Y dwarfs from the 20pc census of 2021ApJS..253....7K. Our targets occupy the bottom of the main sequence, and further highlight the large photometric scatter among the coldest brown dwarfs. Bottom: median absolute Spitzer ch2 magnitude as a function of ch1--ch2 color, in bins of 0.25 mag. The error bars represent the $1\sigma$ scatter in the same bins.
• Figure 5: A comparison between our measured astrometric distances and the photometric distances estimated using the Spitzer magnitudes and the polynomial relations from 2021ApJS..253....7K. The large photometric diversity among cold brown dwarfs makes distance estimates often inaccurate and, in general, of much lower precision compared to astrometric measurements.