Classification for 969 double-mode RR Lyrae stars from Zwicky Transient Facility

TL;DR

RR Lyrae double-mode stars (RRd) enable metallicity-insensitive period–luminosity relations, but sample sizes have been small. The authors develop an automated Lomb-Scargle–based RRd-screening pipeline and apply it to Gaia DR3 RRL cross-matched with ZTF DR22, producing 39,322 RRL with reliable periods and 969 RRd stars, including 614 new RRd detections; completeness against Gaia DR3 RRd is ~47.7%. The RRd sample largely consists of first-overtone-dominated pulsators, and the Petersen diagram reveals a tight sequence for these objects, while a PLR calibrated with Gaia parallaxes shows consistency with the LMC relation, albeit with sizable scatter dominated by parallax errors. The work demonstrates the power of combining space- and ground-based time-domain data to expand the RRd census and improve distance measurements, with anticipated gains from upcoming surveys such as LSST and CSST.

Abstract

RR Lyrae (RRL) variable stars are cornerstone distance indicators. In particular, double-mode RR Lyrae (RRd) stars enable period--luminosity relations (PLRs) that are less sensitive to metallicity, reducing systematic biases in distance measurements. However, their utility has been limited by a global sample of only $\sim$3,000 objects. We develop an automated RRd-screening pipeline and apply it to a cross-matched sample between the Gaia DR3 RRL catalog and ZTF DR22 time-series photometry. The workflow combines Lomb--Scargle period searches, iterative pre-whitening, period-ratio constraints that suppress $\sim$1-day sampling aliases, and amplitude-based quality cuts, enabling large-scale RRd star screening. We produce two ZTF-based catalogs: (i) 39,322 reliable single-mode RRL (40.5\% of the cross-matched set) and (ii) 969 RRd stars. Among the RRd stars, 614 objects are newly identified, substantially enlarging this previously scarce sample; the catalog achieves an estimated completeness of 47.7\%. The PLR derived from the newly discovered RRd stars agrees with the LMC-based relation, though with larger uncertainties. Incorporating these stars will help tighten the RRd PLR and improve distance measurements. Looking ahead, systematic RRd searches with upcoming surveys such as the Legacy Survey of Space and Time (LSST) and the China Space Station Telescope (CSST) should further extend high-accuracy distances across the Local Group and strengthen their cosmological applications.

Figures (5)

• Figure 1: This figure shows an example of LS output results. Top left: power spectrum of the photometric data; Top right: light curve folded with the primary period; Bottom left: power spectrum of the residual data; Bottom right: light curve of the residuals folded with the second period.
• Figure 2: Period-amplitude diagram for RRL. Gray points represent RRab and RRc stars. Red points are RRd stars where the primary period is the fundamental mode; blue points are RRd stars where the primary period is the first-overtone mode. Cross symbols mark the corresponding primary period, triangle symbols mark the second period.
• Figure 3: Left Panel: Petersen diagram for RRd stars screened using ZTF data. Red points represent RRd stars where $P_1$ is the fundamental mode, blue points represent RRd stars where $P_1$ is the first-overtone mode. Blue points form a clear sequence while red points show no clear sequence and may be anomalous RRd stars. Right Panel: Petersen diagram for Gaia DR3 RRd stars with ZTF $r$-band coverage. Blue points: Gaia DR3 RRd subclass after cross-matching with ZTF. Red points: among the cross-matched set, RRd stars recovered by our pipeline where $P_1$ is the first-overtone mode. Green triangles: among the cross-matched set, RRd stars recovered by our pipeline where $P_1$ is the fundamental mode.
• Figure 4: $r$-band magnitude histograms of the Gaia--ZTF cross-matched RRL catalog and the final RRL catalog from this work. The red histogram represents the final ZTF RRL catalog, while the blue one corresponds to the Gaia--ZTF cross-matched RRL sample. The bin size is 0.5 mag, with sources brighter than 13 mag and fainter than 22 mag grouped into single bins at each end. The green line indicates the completeness, defined as the ratio of the red to blue counts in each bin. This figure demonstrates how the catalog completeness varies as a function of magnitude.
• Figure 5: Period-luminosity relation diagram for 90 RRd stars. This diagram is based on the Wesenheit magnitudes from Gaia's G, BP, and RP bands plotted against the fundamental-mode period. The red solid line represents the fitting result of this work, and the black solid line represents the result of 2023NatAs...7.1081C, where the distance modulus of the Large Magellanic Cloud is taken as 18.476 mag 2019Natur.567..200P. The lower panel shows a scatter plot of the residuals from the PLR.