Statistics – Computation
Scientific paper
Jul 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999eso..pres...11.&link_type=abstract
ESO Press Release, 07/1999
Statistics
Computation
Nn Ser
Scientific paper
VLT Observations of an Unusual Stellar System
Reinhold Häfner of the Munich University Observatory (Germany) is a happy astronomer.
In 1988, when he was working at a telescope at the ESO La Silla observatory, he came across a strange star that suddenly vanished off the computer screen. He had to wait for more than a decade to get the full explanation of this unusual event.
On June 10-11, 1999, he observed the same star with the first VLT 8.2-m Unit Telescope (ANTU) and the FORS1 astronomical instrument at Paranal [1]. With the vast power of this new research facility, he was now able to determine the physical properties of a very strange stellar system in which two planet-size stars orbit each other.
One is an exceedingly hot white dwarf star , weighing half as much as the Sun, but only twice as big as the Earth. The other is a much cooler and less massive red dwarf star , one-and-a-half times the size of planet Jupiter.
Once every three hours, the hot star disappears behind the other, as seen from the Earth. For a few minutes, the brightness of the system drops by a factor of more than 250 and it "vanishes" from view in telescopes smaller than the VLT. A variable star named NN Serpentis
ESO PR Photo 30a/99
ESO PR Photo 30a/99
[Preview - JPEG: 400 x 468 pix - 152k]
[Normal - JPEG: 800 x 936 pix - 576k]
[High-Res - JPEG: 2304 x 2695 pix - 4.4M]
Caption to ESO PR Photo 30a/99 : The sky field around the 17-mag variable stellar system NN Serpentis , as seen in a 5 sec exposure through a V(isual) filter with VLT ANTU and FORS1. It was obtained just before the observation of an eclipse of this unsual object and served to centre the telescope on the corresponding sky position. The field shown here measures 4.5 x 4.5 armin 2 (1365 x 1365 pix 2 ; 0.20 arcsec/pix). The field is somewhat larger than that shown in Photo 30b/99 and has the same orientation to allow comparison: North is about 20° anticlockwise from the top and East is 90° clockwise from that direction.
The unsual star in question is designated NN Serpentis , or just NN Ser . As the name indicates, it is located in the constellation of Serpens (The Serpent), about 12° north of the celestial equator. A double letter, here "NN", is used to denote variable stars [2]. It is a rather faint object of magnitude 17, about 25,000 times fainter than what can be perceived with the unaided eye. The distance is about 600 light-years (180 pc).
In July 1988, Reinhold Häfner performed observations of NN Ser (at that time still known by its earlier name PG 1550+131 ) with the Danish 1.54-m telescope at La Silla. He was surprised, but also very pleased to discover that it underwent a very deep eclipse every 187 minutes. Within less than 2 minutes, the brightness dropped by a factor of more than 100 (5 magnitudes). During the next 9 minutes, the star completely disappeared from view - it was too faint to be observed with this telescope. It then again reappeared and the entire event was over after just 11 minutes. Why eclipses are so important for stellar studies
An eclipse occurs when one of the stars in a binary stellar system moves in front of the other, as seen by the observer. The effect is similar to what happens during a solar eclipse when the Moon moves in front of the Sun. In both cases, the eclipse may be partial or total , depending on whether or not the eclipsed star (or the Sun) is completely hidden from view. The occurence of eclipses in stellar systems, as seen from the Earth, depends on the spatial orientation of the orbital plane and the sizes of the two stars. Two eclipses take place during one orbital revolution, but they may not both be observable.
The physical properties of the two stars in a binary system (e.g., the sizes of the stars, the size and shape of the orbit, the distribution of the light on the surfaces of the stars, their temperatures etc.) can be determined from the measured "light-curve" of the system (a plot of brightness vrs. time). The stars are always too close to each other to be seen as anything but a point of light. The light-curve thus describes the way the total brightness of the two stars changes during one orbital revolution, including the variation of the combined light of the two components as they cover each other during the eclipses.
Already in 1988, it was concluded that the eclipse observed in NN Ser must be caused by a bright and hot star (a white dwarf ) being hidden by another body, most probably a red dwarf star . Because of the dramatic effect, this object soon became known as the "Vanishing Star" , cf. ESO Press Release 09/88 (8 December 1988). Critical information missing for NN Ser
One particularly critical piece of information is needed for a light-curve study to succeed, that is whether the eclipse is "total" or "partial" . If during the eclipse one star is entirely hidden by the other, we only see the light of the star in front. In that case, the measured amount of light does not change during the phase of totality. The light-curve is "flat" at the bottom of the minimum and the measured brightness indicates the intrinsic luminosity of the eclipsing star. Moreover, for a given orbit, the duration of the totality is proportional to the size of that star.
This crucial information was not available for NN Ser . The brightness at minimum was simply too faint to allow any measurements of the system with available telescopes during this phase. For this reason, the properties of the eclipsing star could only be guessed. Reaching for the bottom
The new VLT observations have overcome this. Thanks to the powerful combination of the 8.2-m ANTU telescope and the multi-mode FORS1 instrument, it was possible to measure the complete lightcurve of NN Ser , also during the darkest phase of the eclipse.
This extreme observation demanded most careful preparation. Since there is very little light available, the longest possible integration time must be used in order to collect a sufficient number of photons and to achieve a reasonable photometric accuracy. However, the eclipse only lasts a few minutes and it would only be possible to exposure and read-out a few, normal exposures from the CCD camera, not enough to fully characterize the light curve at minimum.
Reinhold Häfner decided to use another method. By having the telescope perform a controlled change of position on the sky ("drift") during the exposure, the light from NN Ser before, during and after the eclipse will not be registered on the same spot of the camera detector, but rather along a line. He carefully chose a direction in which this line would not cross those of other stars in the neighbourhood of NN Ser . This was ensured by rotating FORS1 to a predetermined position angle.
The drift rate was fixed as one pixel (0.20 arcsec) per 3 seconds of time, a compromise between the necessary integration time and desired time resolution that would give the best chance to document the exact shape of the light-curve . In theory, this would then allow the measurement of the intensity along the recorded trail of NN Ser and hence its brightness at any given time during the eclipse.
But how deep would the eclipse be? Would the resulting exposure on each pixel at minimum light be long enough to register a measurable signal? Seeing the light from the cool star!
ESO PR Photo 30b/99
ESO PR Photo 30b/99
[Preview - JPEG: 400 x 464 pix - 156k]
[Normal - JPEG: 800 x 927 pix - 584k]
[High-Res - JPEG: 2292 x 2662 pix - 4.1M]
ESO PR Photo 30c/99
ESO PR Photo 30c/99
[Preview - JPEG: 472 x 400 pix - 48k]
[Normal - JPEG: 943 x 800 pix - 96k]
Caption to ESO PR Photo 30b/99 : 18.5-min "drift" exposure with VLT ANTU and FORS1 of the sky field around the variable stellar system NN Ser (indicated with an arrow). The telescope moved 1 pixel (0.20 arcsec) every 3 seconds so that the images of the stars in the field are trailed from left to right. After some minutes, the very deep eclipse of NN Ser begins when the brightness drops dramatically during the first partial phase. The star is clearly visible at a constant level all through the total phase at
No affiliations
No associations
LandOfFree
A Vanishing Star Revisited does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with A Vanishing Star Revisited, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and A Vanishing Star Revisited will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1621138