The Hybrid Debris Disk Host Star HD 21997 is a High-Frequency Delta Scuti Pulsator

HD 21997 is host to a prototypical"hybrid"debris disk characterized by debris disk-like dust properties and a CO gas mass comparable to a protoplanetary disk. We use Transiting Exoplanet Survey Satellite time series photometry to demonstrate that HD 21997 is a high-frequency delta Scuti pulsator. If the mode identification can be unambiguously determined in future works, an asteroseismic age of HD 21997 may become feasible.


INTRODUCTION
HD 21997, a ∼1.8M ⊙ A3 star, hosts a debris disk bearing a CO gas mass that is large for its age and dust properties (Kóspál et al. 2013;Moór et al. 2013).Although the presence of gas in debris disks is now known in over 20 systems, for a subset of these, including HD 21997, it is so large that it challenges models of second-generation exocometary gas release.These have been termed "hybrid" debris disks due to the potential coexistence of second-generation dust with primordial gas (Kóspál et al. 2013;Kral et al. 2019).Accurate age constraints for HD 21997 are necessary to interpret the dust and gas evolution both for the system and for hybrid disks as a population.In this context, asteroseismic modeling of intermediate-mass delta Scuti (δ Scuti) pulsators has the potential to yield a precise age (e.g., Murphy et al. 2021).We therefore investigate whether HD 21997 is a δ Scuti pulsator.

PULSATION CLASSIFICATION & DISCUSSION
We downloaded the Transiting Exoplanet Survey Satellite (TESS, Ricker et al. 2015) two-minute cadence PDC-SAP SPOC light curve (Smith et al. 2012;Stumpe et al. 2012Stumpe et al. , 2014;;Jenkins et al. 2016) of HD 21997 observed in Sector 4 (2018 Oct 19 -Nov 14).In the resulting amplitude spectrum, we identify 25 significant frequencies spanning ∼49-70 cycles day −1 (Figure 1a) by iteratively fitting sine waves to the time series down to a spectral significance threshold of 20 using Sigspec (Reegen 2007).These frequencies are consistent with pressure modes of δ Scuti stars.Following the procedure described in Sepulveda et al. (2022), in Figure 1b we compare the Gaia DR3 BP − RP color and absolute G magnitude of HD 21997 (Gaia Collaboration et al. 2016, 2021) to that of the population of Kepler δ Scuti stars from Murphy et al. (2019).
Corresponding author: Aldo G. Sepulveda aldo.sepulveda@hawaii.edu2019) and small grey dots are non-pulsating KIC stars.HD 21997, whose color and absolute magnitude were calculated using AV = 0.075 (Chen et al. 2014), is overplotted as a large cyan star.
A precursor to asteroseismic modeling of HD 21997 is an accurate mode identification.By inspecting échelle diagrams, we find plausible large-frequency separation (∆ν) values in the range of 6.13-6.56cycles day −1 , which are all consistent within the range found in Bedding et al. (2020).However, the ambiguity in ∆ν precludes a straightforward mode identification.
Our detection of high-frequency δ Scuti pulsations is independent qualitative evidence for HD 21997's youth (Bedding et al. 2020), which is furthermore supported by its position in our Gaia DR3 color-magnitude diagram.Ujjwal et al. (2020) used isochrone fitting together with Gaia DR2 data of HD 21997 to estimate an age of 38.5 Myr.HD 21997 is an associated member of the Columba young moving group (Torres et al. 2008), with age estimates spanning 20-40 Myr (e.g., Torres et al. 2008;Bell et al. 2015;Ujjwal et al. 2020).These properties support that HD 21997 is likely <50 Myr.Yet, how accurately these ages represent HD 21997 is subject to further inquiry.The isochrone age estimate from Ujjwal et al. (2020) has no reported uncertainty, which would include model systematics from the isochrones used in addition to any statistical uncertainty.In addition, the rotation period of HD 21997 is unknown, and may have a large effect on the star's position on the HR diagram, and therefore also with respect to any isochrones (Pérez Hernández et al. 1999).Moving group members themselves generally have age spreads with respect to the common age adopted for the group, and furthermore moving group ages themselves are subject to systematics of which stars and methods were used for the age constraints.If the asteroseismic mode identification can be achieved for HD 21997 in the future, an asteroseismic age of 10-20% precision may be feasible.

Figure 1 .
Figure 1.(a): TESS amplitude spectrum of HD 21997 zoomed in on the region of significant pulsations.Significant frequencies from our prewhitening procedure are denoted by black downward triangles.(b): Gaia DR3 color-magnitude diagram generated following Sepulveda et al. (2022).Orange diamonds are a sample of Kepler δ Scuti stars from Murphy et al. (2019) and small grey dots are non-pulsating KIC stars.HD 21997, whose color and absolute magnitude were calculated using AV = 0.075(Chen et al. 2014), is overplotted as a large cyan star.