Medium-resolution spectroscopic study of the intermediate-mass pre-main sequence binary $θ^1$ Ori E

TL;DR

This work presents a long-term, high-resolution spectroscopic campaign to characterize the intermediate-mass pre-main-sequence binary θ^1 Ori E, yielding a nearly circular 9.895-d orbit with precise semi-amplitudes and a small systemic velocity. By combining radial-velocity data with Spitzer eclipse constraints, the authors derive accurate dynamical masses ($M_1=2.755\pm0.043\,M_\\odot$, $M_2=2.720\pm0.043\,M_\\odot$), radii ($R_1\approx R_2\approx6.25-6.26\,R_\\odot$), and a common effective temperature ($T_{\rm eff}=5150\pm200$ K), placing both components on very young PMS evolutionary tracks. The results indicate an age $\lesssim 10^5$ yr and suggest the pair consists of the most massive PMS stars with mass precision better than 2%, providing a critical benchmark for intermediate-mass PMS evolution. The analysis also revisits the system’s dynamical state within the Orion Trapezium, concluding that θ^1 Ori E is likely not escaping the cluster, consistent with Gaia DR3 and VLBI proper-motion data. Overall, the paper tightens the empirical constraints on PMS stellar properties and tests PMS evolutionary models at the high-mass end for near-equal-mass binaries.

Abstract

$θ^1$ Ori E is a very young and relatively massive pre-main sequence (PMS) spectroscopic and eclipsing binary with nearly identical components. We analyze Échelle spectra of the system obtained over fifteen years and report 91 radial velocities measured from cross-correlating the observations with a suitable synthetic spectrum. The spectra of individual binary components are indistinguishable from each other, with a composite spectral type around G4 III. The projected equatorial velocity is estimated to be $v \sin{i} = 32\pm 3~km~s^{-1}$, consistent with rotational synchronization. We find that the circular orbit has $P_{\rm orb} = 9.89522 \pm 0.00003~d$, $K_1 = 83.36 \pm 0.29~km~s^{-1}$, $K_2 = 84.57 \pm 0.28~km~s^{-1}$, and $asini = 32.84\pm0.08\ R_\odot$. The mass ratio is $q = 0.9856 \pm 0.0047$, indicating nearly identical but significantly different masses. The systemic velocity of the binary, $γ= 29.7 \pm 0.2~km~s^{-1}$, is similar to that of other Trapezium members. Using Spitzer light curves and our results, we derive $M_1 = 2.755\pm0.043\ M_{\odot}$, $M_2 = 2.720\pm0.043\ M_{\odot}$, $R_1=6.26\pm0.31R_{\odot}$ and $R_2=6.25\pm0.30R_{\odot}$. Together with our estimate of the effective temperature, $T_{\rm eff}=5150\pm200\ K$, a bolometric luminosity of $28.8\pm4.6\ L_{\odot}$ is derived for each component. Compared to evolutionary models of PMS stars, the binary age turns out to be less than or equal to $\sim 10^5$ years. Its components are probably the most massive stars known with masses determined with precision better than 2 percent, with both being PMS stars.

Figures (4)

• Figure 1: Radial velocity curve of $\theta^1$ Ori E. Small, black points are our radial velocity points, while large color circles are those obtained by herfin06, and are not used in our orbital solution. The blue line is the best fit solution for the primary component; the green one is that for the secondary. Residuals (O-C) are shown in the bottom panel. The red error bars correspond to the primary component, while the gray ones to the secondary.
• Figure 2: Temperature versus ratio of spectral depths. In green FeI 5447/FeI 5445, in red FeI 5404/FeI 5406, in blue FeII 5317/FeI 5324. Filled dots are for $log\ g=3.0$ and open dots are for $log\ g=3.5$ in the synthetic spectra. The average value for each ratio as measured in the composite spectra of $\theta^1$ Ori E near conjunction (Table \ref{['tab:LogConj']} is plotted as a square with horizontal and vertical error bars at an interpolated $log\ g=3.3$, and at estimated Temperature values of $5300\ K$ (green), $5020\ K$ (red) and $5000\ K$ (blue), respectively. From this analysis, we adopt $T_{eff}=5150 \pm 200 \,K$ for the average temperature of both nearly identical components of the binary.
• Figure 3: Predicted amplitude of the Rossiter-McLaughlin effect during the primary eclipse of $\theta^1$ Ori E. We show the predicted deviation of the radial velocity curve from the Keplerian orbit during the primary transit with the best-fit radii and inclination from mor12 (solid line) and the upper limit of the deviation (i= 90$^\circ$; dotted line). The radial velocities presented in this paper at conjunction are not sufficiently precise to measure the RM effect.
• Figure 4: Pre-main sequence evolutionary models from Palla1999: We have adapted their Fig. 1 by placing a solid dot at our measurement of $\theta^1~Ori~E$ ($L=24.8\pm 4.6\ L_{\odot}$ and $T_{eff} = 5150 \, \pm \, 200 \ K$). The uncertainty in the luminosity is roughly the size of the point, while the uncertainty in the temperature is much smaller than the symbol. The dotted lines are isochrones that span from 0.5 to 100 Myr. The solid black lines describe the evolution of stars of different masses, where the mass (in M$_{\odot}$) is given where the PMS evolutionary track reaches the Zero-Age Main Sequence. Importantly, the evolutionary tracks start at the 'birth-line', that was empirically defined for the Orion Nebula Cluster in Palla1999, and that describes the moment in the star's PMS stage at which the envelope becomes transparent and thus the star emerges from the molecular cloud.