Deep Search for a scattered light dust halo around Vega with the Hubble Space Telescope (2410.24042v2)

Abstract: We present a provisory scattered light detection of the Vega debris disk using deep Hubble Space Telescope coronagraphy (PID 16666). At only 7.7 parsecs, Vega is immensely important in debris disk studies both for its prominence and also because it allows the highest physical resolution among all debris systems relative to temperature zones around the star. We employ the STIS coronagraph's widest wedge position and classical Reference Differential Imaging to achieve among the lowest surface brightness sensitivities to date ($\sim 4\,\mu Jy/arcsec^{2}$) at wide separations using 32 orbits in Cycle 29. We detect a halo extending from the inner edge of our effective inner working angle at $10^{\prime\prime}.5$ out to the photon noise floor at $30^{\prime\prime}$ (80 - 230 au). The face-on orientation of the system and the lack of a perfectly color-matched PSF star have provided significant challenges to the reductions, particularly regarding artifacts from the imperfect color matching. However, we find that a halo of small dust grains provides the best explanation for the observed signal. Unlike Fomalhaut (a close twin to Vega in luminosity, distance, and age), there is no clear distinction in scattered light between the parent planetesimal belt observed with ALMA and the extended dust halo. These HST observations complement JWST GTO Cycle 1 observations of the system with NIRCam and MIRI.

• The paper achieved one of the lowest-ever surface brightness sensitivities using 32 HST orbits to detect Vega’s dust halo.
• It applied STIS coronagraph imaging with classical reference differential imaging to maximize the signal-to-noise ratio at wide separations.
• The findings reveal a broad halo of small dust grains without a defined belt, contrasting with systems like Fomalhaut and indicating complex disk dynamics.

Overview of the Deep Search for a Scattered Light Dust Halo Around Vega

This paper presents an investigation into the scattered-light detection of the debris disk surrounding Vega, utilizing deep imaging with the Hubble Space Telescope (HST) and its STIS coronagraph. Positioned at 7.7 pc, Vega offers a significant advantage for debris disk studies because of its proximity and brightness. This proximity allows for unparalleled resolution when analyzing temperature zones within the debris disk systems. Employing 32 orbits in Cycle 29 of HST coronagraphy, the researchers achieved one of the lowest-ever surface brightness sensitivities in scattered light observations.

Study Details

The researchers focused on the detection of a dust halo around Vega, which could be observed extending from the inner working angle out to 30″ (80–230 au) from the star. The setup involved using the widest wedge positions of the STIS coronagraph and classical reference differential imaging (cRDI) to maximize the signal-to-noise ratio (S/N) at wide separations. However, the face-on orientation of the disk and absence of a color-matched point-spread function (PSF) star significantly challenged the data reduction, with notable artifacts from imperfect color matching impacting the results.

Key Observations and Comparisons

One central finding of the paper is the detection of a halo composed of small dust grains, but unlike Fomalhaut—a close counterpart to Vega in many stellar characteristics—there’s no defined demarcation between the planetesimal belt detected with the Atacama Large Millimeter/Submillimeter Array (ALMA) and the dust halo. The results emphasize an extended halo that lacks the clearly-defined features observed in other similar stellar systems such as Fomalhaut. The contemporaneous analysis highlights differences in the disk structures, with Fomalhaut displaying a more confined debris belt in scattered light, bolstered by planetary interactions, unlike the broader distribution seen around Vega.

Numerical Results and Analysis

The paper estimates a mean scattering optical depth of approximately $6.7 \times 10^{-6}$ within the region extending from 10.5″ to 27″. Furthermore, it calculates the albedo at a 90-degree scattering angle as ≥0.31, which aligns the observed scattered-light flux levels with the thermal emission properties measured via other waves such as those captured with JWST/MIRI.

Implications and Future Directions

These observations elucidate key differences in the morphology of debris disks within similar stellar environments. Furthermore, they suggest the presence of small dust grains with limited lifetimes that must be continuously replenished through processes that are yet to be fully understood. The findings indicate potential variations in dynamical stirring mechanisms or the absence of planetary shepherding in Vega's system, compared to Fomalhaut.

The gap in knowledge about how such large numbers of sub-blowout grains persist in the Vega debris disk provokes further exploration. This may involve high-resolution observations with next-generation space telescopes and advanced dust modeling techniques to validate scenarios concerning dust transport processes and their origins. Overall, the results underline the complex dynamical processes influencing debris disks and hint towards a need for comprehensive models that can account for observed discrepancies in scattered light among similar stars.

Conclusion

The research presents a meticulous paper of the scattered-light characteristics of the debris disk around Vega and delineates its distinctive structure when compared with other archetype systems. As observational techniques advance and more refined data become available, understanding the nuanced interplay of grain dynamics in debris disks will remain a pivotal area of research, fostering deeper insight into circumstellar environmental conditions across different planetary systems.