Download preprint accepted by Astrophysical Journal Letters on 22 Apr, 2005 (880Kb PS file or 468Kb PDF file).

Download ASCII file of photometric data (from the STARE instrument of the TrES Network) or radial velocity data (from the High-Resolution Spectrograph of the Hobby-Eberly Telescope). To download, click on right mouse button and choose 'Save Target As'.

A New Detached M Dwarf Eclipsing Binary Observed by the TrES Network

The TrES network observed a field in the constellation of Hercules during May and June 2003. During an analysis of the combined photometry from all three telescopes, an interesting stellar object was found. At first it was suspected of being a transiting planet, until further investigation showed that this was actually an eclipsing binary system with two very similar components. From 2MASS colours, we estimated the system to consist of two low mass objects. We then observed this object to obtain radial velocity measurements which would give us information about the masses of the individual objects. In September 2004, the system was confirmed (using the Hobby-Eberly Telescope, Texas) to consist of two M3 Dwarf stars separated by a distance of 0.02 AU.

Why is this important?
As it turns out, there were only four such systems previously observed. So we had turned up the fifth. Detached spectroscopic eclipsing systems are of extreme value, because they permit us to obtain accurate and precise information about some fundamental properties of stars, such as mass and radius. This is not possible using single isolated stars.

Low mass stars such as these are probably the most numerous of objects in our galaxy, however, because they are so faint, they are extremely difficult to detect and study. Finding a system consisting of two of these objects is advantageous because firstly, we obtain information from two stars, and secondly, due to the nature of the system, we can well constrain the errors in this information. Constraining these errors is of particular importance because we can then use them to compare with theory. At this low mass range, we find that they have quite complex transport processes and atmospheres and we don't understand their theory as well as we would like (see figure 2).

Figure 1. The folded photometric light curve of this object. It is folded at its photometric period of 1.12079 days. Phase = 0 corresponds to center of secondary eclipse (when the smaller star is being eclipsed), phase = 0.5 corresponds to center of primary eclipse. The small crosses are the data from the STARE instrument. The continuous line is a model fit to this data.

The second panel shows the residuals after subtracting the model fit from the data. No evidence of eclipse in this residual plot indicates an adequate model fit.

Figure 2. The Mass-Radius Diagram for these low mass observed objects. The triangles indicate the other four known binary systems on this diagram, the squares indicate where our object sits. We have also included some of the small sample of other M dwarf stars (crosses) whose companion is not an M Dwarf, but rather a star of spectral type F or G.

The three lines indicate theoretical models of these stars. There is a discrepancy between the observations and the theory. We hope that the addition of our well contrained object will prove valuable in helping to understand the difference between the theory and observations.

Binary System Info:
Position: 16 50 20.7 +46 39 01 (J2000)
R mag: 14.4, V mag: 15.51, J-K: 0.89
Spectral Types: M3V
Period of orbit: 1.12079 days
Orbit inclination: 82.2 degrees
Semi-major axis: 0.0105 AU
Mass(1) : 0.493 Mass(2) : 0.489 x the mass of the sun
Radius(1) : 0.483 Radius(2) : 0.480 x the mass of the sun