Follow-up of three exocomet-host candidates

TL;DR

This study tests the exocomet hypothesis for three candidate hosts by combining high-resolution Ca II K spectroscopy and infrared interferometry. Weighted Ca II K profiles yield equivalent widths and column densities, while VLTI/PIONIER confirms that HD 85905 is a nearly equal-flux binary, prompting a revision of the stellar parameters to a two-component system with Teff ≈ 9010–9105 K and L_total ≈ 112.6 L⊙. HD 85905 shows pronounced, multi-component variability that could be FEB-like, but the binary architecture complicates attribution; HD 36546 and HD 42111 exhibit only modest variability, leaving the exocomet hypothesis unresolved for these stars. The work highlights the necessity of long-term, multi-technique monitoring (spectroscopy, interferometry, and photometry) to distinguish exocometary signatures from binary or disk-related phenomena and to robustly establish exocomet activity in these systems.

Abstract

Exocomets are small bodies that evaporate when they approach their host star. They are detected through variability of non-photospheric features with spectroscopy and/or asymmetric transits in time-series photometry. In the past four decades ~30 systems have shown such variations, and were therefore classified as exocomet host stars. However, some publications have pointed out mechanisms that might mimic exocometary features, and therefore, careful monitoring is needed to confirm the origin of the observed variability. With this paper we aim to investigate the exocomet nature of the non-photospheric variable features observed in the exocomet candidate stars HD 36546, HD 42111 and HD 85905. All of them have shown some degree of variability, particularly in their Ca II K line. We analysed the non-photospheric Ca II K line features from high-resolution spectra obtained using new NOT/FIES and Mercator/HERMES, and some additional archival spectra of the target stars. The variability was quantified through the changes in the equivalent widths of those features, which are assumed to be of circumstellar origin. Column densities were also estimated for each variable feature. Strong variability was found for HD 85905, consistent with a potential link to exocometary activity. However, the binarity of the system, which we confirmed through interferometric VLTI/PIONIER observations, complicates the interpretation of these signatures and prevents us from drawing definitive conclusions. The remaining two sources do not show any significant variability, but due to the sporadic nature of the exocometary events, we cannot discard the exocomet hypothesis. Further monitoring of the stars will be necessary to carry out a robust determination of the variability patterns and timescales that would completely rule out other scenarios.

Figures (11)

• Figure 1: Left. Squared visibilities as a function of wavelength, obtained for the three observing blocks (three panels) and six baselines (depicted with six different symbols and colours), along with the best-fit binary model described in Sect. \ref{['sec:binarity']} (solid lines). Right. Same as left, for the four closure phases obtained in each observing block. Some of the squared visibilities and closure phases are not displayed for the third observing block, and not used in this analysis, due to lower data quality.
• Figure 2: Weighted mean profiles of the Ca ii K absorption line of HD 36546, HD 42111 and HD 85905 using all the available spectra for each object. The circumstellar non-photospheric narrow components are very distinguishable from the photospheric absorptions.
• Figure 3: Upper panel: Weighted mean of the spectra of HD 36546, obtained on 13 Jan 2023, around the Ca ii K line (blue), and the spline fitting the photospheric profile of the weighted mean spectrum built from all the available observations (red dashed line). The vertical grey dashed line marks the radial velocity of the star. Lower panel: Ratio between the weighted mean spectrum from that date and the spline. The two vertical orange dashed lines mark the limits of the interval where the non-photospheric absorption is detected.
• Figure 4: Maximum variation in the circumstellar line profiles of the stars with respect to their corresponding mean spectrum. For HD 36546 and HD 42111, the insets in both panels show tiny differences between the circumstellar features. For HD 85905, there is a remarkable feature around 25 km/s, similar to those observed in Beust90, making it compatible with the FEB scenario.
• Figure 5: Normalized $\chi^2$ map of the combined $V^2$+CP analysis for the VLTI/PIONIER observation. The white star at the centre marks the (fixed) position of the primary star in the binary model, while the black circle indicates the position of the minimum in the map, i.e., the most likely position for the companion.