Probing Compact Objects in Wide-Orbit Binaries with Joint LAMOST LRS and MRS

TL;DR

The paper develops a time-domain spectroscopic approach by merging LAMOST LRS and MRS catalogs to hunt for compact objects in wide binaries with long orbital periods. It uses Lomb-Scargle periodograms and a mass-function-based screening to identify 76 SB1 candidates with $P_{\rm orb}\sim 10$–$10^{3}$ days, $K_1 \lesssim 50$ km s$^{-1}$, and $f(M_2) \sim 0.05$–$0.60\,M_\odot$, classifying them into three groups by mass ratio and donor evolution. Template-matching with MARCS spectra and MCMC RV extraction, followed by Keplerian orbital fits, yields robust orbital parameters and $f(M_2)$ values; 16 of the targets have Gaia DR3 NSS orbital solutions in agreement with the LAMOST results. The work confirms the value of long-baseline spectroscopy for uncovering quiescent compact objects in wide orbits and anticipates improved inclinations and object masses with future Gaia data releases. Collectively, this study expands the census of wide-separation binaries containing compact remnants and informs binary evolution pathways in the Galaxy.

Abstract

Wide-orbit binaries serve as crucial laboratories for understanding stellar evolution and identifying quiescent compact objects. In this work, we search for compact objects in wide-orbit binaries by merging the LAMOST multi-epoch catalogs from LRS and MRS in the 12th data release. We specifically focus on sources with at least 20 observation epochs that clearly exhibit long-term radial velocity (RV) variations while remaining stable over short time scales. By constraining the mass function with Lomb-Scargle periods and RV ranges, we identified 76 single-lined spectroscopic binary candidates harboring potential compact objects with robust orbital solutions. These systems exhibit orbital periods ranging from 10 to 1000 days, with semi-amplitudes of velocity $K_1 \lesssim 50$ km/s and mass functions $f(M_2)$ between 0.05 and 0.6 $M_{\odot}$. Combining $f(M_2)$ with SED-derived stellar parameters, we identify 6 strong compact object candidates with main-sequence companions (Class A), 24 systems likely consisting of either compact objects with giant/subgiant companions or mass-inverted Algol-type binaries (Class B), and 46 candidates with relatively lower mass ratios (Class C). Cross-matching with the Gaia DR3 nss_two_star_orbit catalog yields 16 sources, all of which exhibit orbital solutions consistent with our results. This study demonstrates the essential role of long-term spectroscopic monitoring in searching for compact objects in wide-orbit binaries and validating orbital solutions. The strategy of leveraging extended time baselines will be increasingly effective as spectroscopic databases continue to grow, enabling the systematic discovery of compact objects in wide orbits across the Galaxy.

Figures (5)

• Figure 1: Flowchart of the sample selection and classification process.
• Figure 2: Orbital characteristics of the 76 candidates in the $K_1$--$P_{\rm orb}$ plane. Contours of constant mass function $f(M_2)$ (at $e=0$, from 0.02 to 0.40 $M_\odot$) are shown as colored lines. Colored circles indicate our candidates, classified into Class A (orange), B (yellow), and C (teal) according to the criteria described in Section \ref{['sec:classification']}. These markers are consistent with Figure \ref{['fig:fm_density']}. We show candidates in L24 (white circles) alongside our new candidates. Notably, the L24 candidates are concentrated at significantly shorter orbital periods, whereas our sample extends to longer orbital periods.
• Figure 3: Distribution of the mass function ratio $f(M_2)/M_1$ versus the average density of visible companion $\rho_1$ for our sample of 76 candidates. We classify our candidates into three distinct categories based on their mass functions, mass ratios, and the evolutionary status of the primary stars. Class A (orange) represents systems with mass functions exceeding $0.138\,M_1$ (corresponding to a mass ratio $q = M_2/M_1 > 0.75$) and a primary star mean density $\rho_1 \approx M_1 R_1^{-3} < 1/20\,\rho_\odot$; these criteria specifically isolate systems consisting of a main-sequence star and a potentially massive compact companion. Class B (yellow) includes targets that exhibit high mass ratios similar to Class A but are characterized by visible primaries in the post-main-sequence stage, such as subgiants or giants, as inferred from their atmospheric parameters. Finally, Class C (teal) comprises the remaining targets with lower mass functions ($f(M_2)/M_1 \le 0.138$) or less constrained orbital parameters, typically associated with lower-mass stellar companions.
• Figure 4: Phase-folded radial velocity curves and best-fit Keplerian orbital solutions for nine representative binary candidates from our sample. For each panel, blue circles denote the MRS observations, orange circles denote the LRS observations. The solid orange line indicates the median fit, while the shaded region represents the bundle of possible orbits consistent with the posterior distribution.
• Figure 5: Representative SED fitting results and posterior probability distributions (corner plots) for two candidates. In the SED panels, the black line represents the best-fit model, and the colored points denote the observed photometry. The corner plots show the correlations between the derived stellar parameters ($T_{\rm eff}$, $R_1$, [Fe/H], and $\log g$).