Observations from Australasia using the Gravitational Microlensing Technique

Philip Yock
, PASA, 17 (1), 35.
Next Section: Extra-Solar Planets
Title/Abstract Page: Observations from Australasia using
Previous Section: Stellar Spectra
Contents Page: Volume 17, Number 1

Subsections

Stellar Atmospheres

The study of stellar atmospheres by the gravitational microlensing provides an interesting application of the technique. Two methods are possible. Examples of each are given below.

MACHO-95-BLG-30

During July 1995 the MACHO group reported a microlensing event in progress with unusual properties. The star being lensed was a red giant, and the lens trajectory was predicted to transit it. Photometric and spectroscopic observations of the event were requested from several locations, and these were carried out (Alcock et al 1997b). The light curve for the event is shown in Figures 2 and 3. These include data from Australia, Chile, Israel and New Zealand.

Figure 5: Network of telescopes operated by the PLANET group.
\begin{figure} \begin{center} \psfig{file=fig5.ps,height=8cm} \end{center} \end{figure}

The above data for event MACHO-95-BLG-30 clearly show the effect of the finite size of the source star. As the lens transits the face of the source star, different parts of it are preferentially amplified. Limb darkening and stellar spots on the source star may then be detected (Heyrovsky, Sasselov & Loeb 1999). For event MACHO-95-BLG-30 an improved fit to the data was in fact obtained with a limb darkened model of the star. This is shown in Figure 3. Spectra taken during the event, shown in Figure 4, also showed some variation as the lens transited the source star. These are presently being analysed for the purpose of constructing a model atmosphere of the source star in this event (Heyrovsky & Sasselov 1999).

Figure 6: Light curve by the PLANET group for event MACHO-98-SMC-1. The data are from SAAO 1-m (circles), the CTIO 90-cm (squares), the CTIO-Yale 1-m (triangles), and the Canopus 1-m (asterisks) telescopes. The inset covers 0.6 days, corresponding to less than one tick mark on the main figure. The data are binned by day on the main figure. Two fits to the data are shown. These are discussed in Albrow et al. (1999). As is apparent from the figure, the models have similar crossing times.
\begin{figure} \begin{center} \psfig{file=fig6.ps,height=9cm} \end{center} \end{figure}

MACHO-98-SMC-1

This event illustrates the second technique by which stellar atmospheres may be probed using gravitational microlensing. MACHO-98-SMC-1 was monitored by several groups including the PLANET group (Albrow et al. 1999a). This group operates the network of telescopes shown in Figure 5. It enables almost continuous surveillance around the clock, weather permitting, of microlensing events. The lens for MACHO-98-SMC-1 was a binary star. Binary lenses can produce light curves that differ markedly from the single peaked structure shown in Figure 2 for a single lens. Figure 6 shows the light curve obtained by the PLANET group for MACHO-98-SMC-1. The asymptotic peaks occur when the source star crosses a 'caustic'. This is a linear region formed by the binary lens where the amplification is formally infinite (Alcock et al. 1999). The shape of the caustic determined by the PLANET group for MACHO-98-SMC-1 is shown in Figure 7.

Figure: Caustic crossing geometry of event MACHO-98-SMC-1 as determined by Albrow et al. (1999a). The diamond-shaped curve is the caustic. The thick solid line and the two thinner parallel lines indicate the trajectory and size of the source star. The two components of the binary are shown by circles whose relative sizes are proportional to their masses. The tick marks are in units of the Einstein crossing time $\hat{t}$ = 108.4 days.
\begin{figure} \begin{center} \psfig{file=fig7.ps,height=7cm} \end{center} \end{figure}

The formally infinite amplification at a caustic is suppressed by the finite size of the source star. As the caustic sweeps across the face of the source star, different regions are preferentially amplified. This has two consequences. First, the crossing time can be measured and used to constrain the geometry of the event. Second, limb darkening in the source star can be detected. The first of these goals was achieved by the PLANET group for MACHO-98-SMC-1. They found a crossing time of $\sim$ 4.25 hr for the event, implying the lens was in the SMC (Albrow et al. 1999a). The event was an example of the so-called 'self-lensing' process described in section 2, and the lens was not in the Galactic halo. By combining the PLANET data for MACHO-98-SMC-1 with data from the EROS, MACHO, GMAN and MPS groups, a detection of limb darkening for the source star in five passbands ranging from I to V was obtained (Afonso et al. 1999b). Spectroscopy and photometry of the source star yielded an A6 dwarf classification. Thus, in this instance, limb darkening on a dwarf star some 60 kpc distant was detected! This highlights the extraordinary capability of the microlensing technique. Because the source star for this event was in the SMC, it is likely to be metal-poor. This represents the first measurement of limb darkening of an A type metal-poor star.
Next Section: Extra-Solar Planets
Title/Abstract Page: Observations from Australasia using
Previous Section: Stellar Spectra
Contents Page: Volume 17, Number 1

Welcome... About Electronic PASA... Instructions to Authors
ASA Home Page... CSIRO Publishing PASA
Browse Articles HOME Search Articles
© Copyright Astronomical Society of Australia 1997
ASKAP
Public