Download Spectroscopic Investigation of Companion Stars in Herbig

Spectroscopic Investigation of Companion Stars in Herbig AeBe Binary Systems
Anne Sweet1, B. Rodgers2, G. Doppmann2, N. van der Bliek3, S. Thomas3, M. J. Cordero1 1CTIO REU, Chile, 2Gemini Observatory, Chile, 3CTIO, Chile.
Abstract
Herbig AeBe (HAEBE) binary systems are good environments for the study of pre-main sequence stellar evolution in companion stars whose mass may be significantly lower than that of the primary star. Measurements of
the spectral type and surface gravity of the companion star in the system allow it to be placed on the H-R diagram, where theoretical evolutionary model tracks can then constrain its mass and age, and comparisons can be
made between these low mass stars and those formed without the presence of a high mass star. Because of the extinction associated with objects in star forming regions, the near-infrared offers a less obscured wavelength
region than the optical through which to study these objects. Medium (R=1,800 & 6,000) and High (R=18,000) Resolution near-infrared (NIR) spectra were gathered for the analysis of these companion stars. We present two
different ways to measure Teff and estimate log g from the spectra of late type stars, depending on the spectral resolution. At high resolution, detailed model fits to the shapes of the Na lines at 2.2 microns and the (2-0)
12CO bandhead at 2.29 microns provides an accurate way to measure effective temperature and surface gravity, in addition to allowing for values of vsini, veiling, and radial velocity to be estimated. At medium resolution,
the equivalent widths of 10 of the strongest absorption lines present in the NIR spectrum were measured to determine Teff at a lower accuracy. Preliminary results show that these techniques are effective at probing the
physical state of the secondary. Analysis of two stars is discussed; more data is needed to address statistical questions about the nature of HAEBE companions.
Our data:
Stars were chosen to be the known binary companion to a Herbig star, with a binary separation greater than 0.8 arcseconds to avoid any
contamination from the primary star. Data was collected using the Gemini South GNIRS instrument (PI Rodgers).Two types of spectral
measurements were made, high resolution (R=18,000) and lower resolution (R=6,000 and R=1,800). All data were reduced following standard
procedures with IRAF from the Gemini GNIRS package.
Why are these systems important?
HAEBE stars are often found in groups and thus these environments are important, not only because binary fraction is critical for the Initial Mass
Function but also because the study of star formation of more massive stars offers insight into stellar evolution as a function of mass. In addition,
learning the properties of stellar evolution of a less massive star in the presence of a higher mass star lends insight into the effect of a binary
environment on stellar formation. Finally, in binary systems there is the possibility of a dynamical mass measurements, which can test theoretical
evolutionary model tracks as well as providing a way to calibrate systematic errors in converting Teff and logg into mass and age.Specifically, in this
work we are interested in the effects that a more massive PMS Herbig star has on the environment and evolution of its low mass companion, which
we explored through two analysis techniques.
High Resolution Analysis:
The high resolution data was analyzed by matching spectral
synthesis models to find the best pixel to pixel fit across the
absorption lines that were present in the data (refer to figure 1)
(Doppmann et al., 2003). From this we could infer Teff and log
g, vsini rotation and radial velocity.We used this technique on
the six standard stars to calibrate our techniques and on BF
Ori B and HR599B.
Lower Resolution Analysis: Going Further
For a more quantitative exploration of the lower resolution data, we also identified which objects showed late-type
features and selected the 10 strongest absorption features on which to base our spectral analysis. An equivalent width
technique developed in this work was used to compare equivalent widths of the observed lines with those same lines in
the NEXTGEN spectral synthesis grid (Hauschildt 2006, private communication) across a range of temperatures and at
2 different logg values (3.5, 4.5). Using an IDL program written by G. Doppmann, equivalent widths were measured
consistently line by line.
Lower Resolution Analysis:
The lower resolution data involved applying a qualitative eyefit to the entire spectrum paying particular attention to the fits
between several strong lines in the spectra and the model
(refer to figure 2). This technique allowed us to determine Teff
and log g. This technique was used on BF Ori B.
Fig 3: Showing the results of the equivalent plots for the
Aluminum lines in our spectrum. Model values for 3.5 and 4.5
are green and blue respectively, red lines are the equivalent
widths of the data and the associated error.
Fig 4: Showing the results of the equivalent plots for the
Sodium and Titanium lines in our spectrum. Color standards
are the same as in figure 3.
Conclusions
Compiling the results from these various techniques and
correcting for the systematic offset determined from the
analysis of the standard stars, we can plot these points on
an H-R diagram (refer to figure 5). Combined with
theoretical evolution tracks, this can give us an idea of mass
and age. We found BF Ori B to have a mass of .4 Msolar
and an age of around 40 Myr, and HR5999 B to have a
mass of .1 Msolar and an age of around 40 Myr.While there
are errors involved and there is still much to be determined,
the preliminary results show that these two stars are late
type and occupy the same region of the graph as the T
Tauri stars. This suggests that the low mass stars formed in
the presence of a massive star may not be very different
from those found alone.
Fig. 1: An example of the high resolution fitting technique used on the
standard star HD74137. Vertical lines denote the region where the RMS
is computed and are fit around the sodium lines (2.07 and 2.09 microns)
and the (2-0) 12CO bandhead (2.2935 microns). This technique allowed
for a precise temperature, vsini rotation, radial velocity, and continuum
veiling measurement (at a fixed gravity) to be made.
Fig 2: An example of the qualitative eye fit plot as part of the medium
resolution fitting technique. Plot shows the sodium lines in the third order
of the spectra of BF Ori B. The solid line denotes the data, and the
dashed line is that of the models. From top to bottom, model spectra
increase in temperature from 3200 K to 4000K at a fixed surface gravity of
log g =3.5. Changes in the model line strengths can be seen with the
stronger lines.This technique allows for a more comprehensive fit to Teff
and veiling, but without precise kinematic information,
Future work includes the analysis of more stars with high
resolution data of both the Na and CO spectral regions as
well as the refinement of the medium resolution equivalent
width technique to better constrain the degeneracy between
Teff and log g.
Figure 5: Our results for BF Ori B and HR5999 B,
corrected for the systematic offset in our fitting
techniques. These stars lie in the H-R diagram in
the same region that the low mass T Tauri stars do.
References
Doppmann, G.W., Jaffe, D.T., White, R.J., 2003, AJ, 126, 3043-3057
Hauschildt 2006, private communication
Rodgers, B. Gemini South Observing Proposal, semester 2006A
We thank the National Science Foundation for the support of this REU program under grant no. 0353843.