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Extragalactic novae
New Astronomy Reviews 44 (2000) 87–91 www.elsevier.nl / locate / newar
Extragalactic novae Michael M. Shara 1 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
Abstract Novae are expected to form in all stellar systems with a binary population. Detection of extragalactic novae provides direct evidence of close binary populations and possible spatial variations in those populations. Comparison of extragalactic novae with their local counterparts can yield valuable tests of close binary evolution theory. I report early results from surveys of globular clusters, the Large Magellanic Cloud and M81 for classical novae in eruption and in quiescence. T Sco, the nova of 1860 A.D. in the globular cluster M80, has now been recovered. It is three magnitudes fainter than canonical old novae, though this might be an inclination effect. Seven quiescent old novae in the Large Magellanic Cloud have been recovered (at brightnesses comparable to their Galactic counterparts). Their orbital periods are now within reach. Twenty-three novae have been detected on archival 5 meter Palomar plates of M81. The spatial distribution of these novae strongly suggests that most come from the spiral arm population. 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Binary stars of intermediate initial mass and separation will produce the mass transferring white dwarf – red dwarf (WD-RD) pairs that are the underlying systems of classical novae. There is no a priori reason to expect close binary formation or evolution to differ dramatically from galaxy to galaxy or with redshift. Thus novae are expected to form in all stellar populations and at all epochs. Furthermore, since the WDs (and NOT the RDs) in nova binaries are responsible for the high metal enrichments in novae ejecta, novae in high-Z and low-Z systems should not differ significantly in their eruptive properties. The orbital period distribution of novae is predicted to be only mildly dependent on host population metallicity. These theoretical beliefs, the products of a generation of increasingly sophisticated modeling, are largely untested, and should be taken with much salt. 1
Now at Department of Astrophysics, American Museum of Natural History, CPW at 79th st., New York, NY 10024, USA
The observational programs described below are aimed at testing these widely believed but empirically unsupported claims.
2. Novae in globular clusters Two classical novae have been sighted in globular clusters in the past 140 years. The RD companions must be as metal poor as any known in the galaxy, so the recovery and characterization of these systems is a potentially important test of the metallicity claims noted above. Only the advent of the Hubble Space Telescope (HST) has made this possible; the cores of globulars are very crowded and the quiescent novae are very faint. In Fig. 1, I show a montage of HST images of the field of T Sco, the classical nova seen in 1860 A.D. near the center of the globular cluster M80 (NGC 6093). Accurate astrometry (2 arcseconds) of the nova relative to nearby red giants, carried out in 1860, leads to the small error circle (determined by Laurent Drissen) shown in the figure. Remarkably,
1387-6473 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S1387-6473( 00 )00019-1
88
M.M. Shara / New Astronomy Reviews 44 (2000) 87 – 91
the bluest object in the cluster (of | 16000 stars) falls in the error circle. There is little doubt that this is the quiescent old nova. In Fig. 2, I show the HST color magnitude diagram (CMD) of M80 with the old nova marked as a triangle. Two dwarf novae (DN) were discovered in M80 during our multi epoch survey. Their bright and faint states are also plotted in the M80 CMD. Absolute magnitudes of cataclysmic binaries are notoriously
difficult to determine. Here, for the first time, we have a direct comparison of DN and CN at precisely the same distance. Local quiescent DN are typically three magnitudes fainter than quiescent CN. Why is T Sco so faint? It is possible that we are seeing T Sco highly inclined; Brian Warner has taught us how important an effect this can be. It is also possible that T Sco is headed towards a low mass transfer (hibernation) state. Detailed characterization of these
Fig. 1. Montage of HST / WFPC2 images of the the field of T Sco. The position of T Sco is circled. Note it’s brightness in the UV images and lack of detection in the red filters. The exposure time of F255W is not as long as F336W or F160BW and explains the failure to convincingly detect T Sco in this image.
M.M. Shara / New Astronomy Reviews 44 (2000) 87 – 91
89
rare but precious globular cluster cataclysmics will yield valuable insights into the long-term evolution of CVs.
3. Quiescent old novae in the Large Magellanic Cloud
Fig. 2. The HST / WFPC2 F336W (U) - F675W (R) color magnitude diagram of M80 (NGC 6093). T Sco marked with a triangle. Two dwarf novae also in the field are plotted with thier magnitudes at quiesence and eruption.
The rarity of globular cluster novae precludes using them for determining the absolute magnitudes of post eruption systems. Fortunately, about two dozen classical novae have been detected during the last century in the Large Magellanic Cloud (LMC). Unfortunately, the quiescent objects are expected to be very faint (23 to 25 magnitude) and very crowded. To make matters worse, the published positions of the early novae are very poor, typically 61arcminute. Dozens of faint, UV bright objects exist in each square arcminute of the LMC. To make headway I have assembled all the available early archival plates of erupting LMC novae. The positions of these novae were determined to accuracies of 1 to 5 arcseconds (depending on the plate scale of the discovery plates). Deep CCD photometry in U, B, and V of almost all LMC nova fields has been carried out in collaboration with John Graham. Seven good candidates have been located (and twice as many fields have no obvious candidates to the plate limits of U | B |V | 24). Fig. 3
Fig. 3. Finding charts for LMC 1948. The 3-sigma positional error is marked by the circle. LMC 1948 is located on the left hand side (around 8 o’clock) between a bright star and the error circle.
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M.M. Shara / New Astronomy Reviews 44 (2000) 87 – 91
Fig. 4. Finding charts for LMC 1971b. The 3-sigma positional error is marked by the circle. LMC 1971b is not detected in these images.
Fig. 5. KPNO 0.9m B image of M81. The positions of the novae are indicated and numbered on the image.
M.M. Shara / New Astronomy Reviews 44 (2000) 87 – 91
shows the finding chart for LMC 1948 A.D., a clear detection. Fig. 4 is the chart for LMC 1971b, a clear nondetection. While the CCD plate limits in this survey are 1 to 2 mag fainter than the expected brightnesses of canonical old LMC novae, I don’t place much significance on the modest recovery fraction. The luminosity distribution of the LMC old novae could be compatible with that of the Galactic old novae. This is because crowding is severe in many of the fields, and it is likely that HST and / or 8 meter ground based imagery will be required to recover most of the rest of the quiescent LMC cataclysmics. The recovered objects’ orbital periods are now within reach. This will be an important test of the close binary evolution prediction that this period distribution is similar to that of the Galactic novae.
4. Novae in M81 A five year series of Palomar 5 meter photographic plates of M81 have been analyzed (in
91
collaboration with Allan Sandage and David Zurek). Twenty three novae were detected: they are shown in Fig. 5. The key result of the survey is the remarkable spatial distribution of the erupting cataclysmic binaries. Despite the incompleteness of nova detections in the central arcminute of the galaxy (due to the modest dynamic range of the photographic plates) the prevalence of novae in the spiral arms is evident. This is consistent with theoretical suggestions that massive WDs, descended from relatively massive primaries in spiral arms, should erupt as novae much more frequently than novae with low mass WD primaries.
5. Summary Extragalactic novae are challenging to detect and to characterize. They have much to teach us about cataclysmic binary host populations and stellar evolution. The coming generation of 8 meter telescopes, in conjunction with HST and other space telescopes are the essential tools for carrying out these studies.
Extragalactic novae Michael M. Shara 1 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
Abstract Novae are expected to form in all stellar systems with a binary population. Detection of extragalactic novae provides direct evidence of close binary populations and possible spatial variations in those populations. Comparison of extragalactic novae with their local counterparts can yield valuable tests of close binary evolution theory. I report early results from surveys of globular clusters, the Large Magellanic Cloud and M81 for classical novae in eruption and in quiescence. T Sco, the nova of 1860 A.D. in the globular cluster M80, has now been recovered. It is three magnitudes fainter than canonical old novae, though this might be an inclination effect. Seven quiescent old novae in the Large Magellanic Cloud have been recovered (at brightnesses comparable to their Galactic counterparts). Their orbital periods are now within reach. Twenty-three novae have been detected on archival 5 meter Palomar plates of M81. The spatial distribution of these novae strongly suggests that most come from the spiral arm population. 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Binary stars of intermediate initial mass and separation will produce the mass transferring white dwarf – red dwarf (WD-RD) pairs that are the underlying systems of classical novae. There is no a priori reason to expect close binary formation or evolution to differ dramatically from galaxy to galaxy or with redshift. Thus novae are expected to form in all stellar populations and at all epochs. Furthermore, since the WDs (and NOT the RDs) in nova binaries are responsible for the high metal enrichments in novae ejecta, novae in high-Z and low-Z systems should not differ significantly in their eruptive properties. The orbital period distribution of novae is predicted to be only mildly dependent on host population metallicity. These theoretical beliefs, the products of a generation of increasingly sophisticated modeling, are largely untested, and should be taken with much salt. 1
Now at Department of Astrophysics, American Museum of Natural History, CPW at 79th st., New York, NY 10024, USA
The observational programs described below are aimed at testing these widely believed but empirically unsupported claims.
2. Novae in globular clusters Two classical novae have been sighted in globular clusters in the past 140 years. The RD companions must be as metal poor as any known in the galaxy, so the recovery and characterization of these systems is a potentially important test of the metallicity claims noted above. Only the advent of the Hubble Space Telescope (HST) has made this possible; the cores of globulars are very crowded and the quiescent novae are very faint. In Fig. 1, I show a montage of HST images of the field of T Sco, the classical nova seen in 1860 A.D. near the center of the globular cluster M80 (NGC 6093). Accurate astrometry (2 arcseconds) of the nova relative to nearby red giants, carried out in 1860, leads to the small error circle (determined by Laurent Drissen) shown in the figure. Remarkably,
1387-6473 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S1387-6473( 00 )00019-1
88
M.M. Shara / New Astronomy Reviews 44 (2000) 87 – 91
the bluest object in the cluster (of | 16000 stars) falls in the error circle. There is little doubt that this is the quiescent old nova. In Fig. 2, I show the HST color magnitude diagram (CMD) of M80 with the old nova marked as a triangle. Two dwarf novae (DN) were discovered in M80 during our multi epoch survey. Their bright and faint states are also plotted in the M80 CMD. Absolute magnitudes of cataclysmic binaries are notoriously
difficult to determine. Here, for the first time, we have a direct comparison of DN and CN at precisely the same distance. Local quiescent DN are typically three magnitudes fainter than quiescent CN. Why is T Sco so faint? It is possible that we are seeing T Sco highly inclined; Brian Warner has taught us how important an effect this can be. It is also possible that T Sco is headed towards a low mass transfer (hibernation) state. Detailed characterization of these
Fig. 1. Montage of HST / WFPC2 images of the the field of T Sco. The position of T Sco is circled. Note it’s brightness in the UV images and lack of detection in the red filters. The exposure time of F255W is not as long as F336W or F160BW and explains the failure to convincingly detect T Sco in this image.
M.M. Shara / New Astronomy Reviews 44 (2000) 87 – 91
89
rare but precious globular cluster cataclysmics will yield valuable insights into the long-term evolution of CVs.
3. Quiescent old novae in the Large Magellanic Cloud
Fig. 2. The HST / WFPC2 F336W (U) - F675W (R) color magnitude diagram of M80 (NGC 6093). T Sco marked with a triangle. Two dwarf novae also in the field are plotted with thier magnitudes at quiesence and eruption.
The rarity of globular cluster novae precludes using them for determining the absolute magnitudes of post eruption systems. Fortunately, about two dozen classical novae have been detected during the last century in the Large Magellanic Cloud (LMC). Unfortunately, the quiescent objects are expected to be very faint (23 to 25 magnitude) and very crowded. To make matters worse, the published positions of the early novae are very poor, typically 61arcminute. Dozens of faint, UV bright objects exist in each square arcminute of the LMC. To make headway I have assembled all the available early archival plates of erupting LMC novae. The positions of these novae were determined to accuracies of 1 to 5 arcseconds (depending on the plate scale of the discovery plates). Deep CCD photometry in U, B, and V of almost all LMC nova fields has been carried out in collaboration with John Graham. Seven good candidates have been located (and twice as many fields have no obvious candidates to the plate limits of U | B |V | 24). Fig. 3
Fig. 3. Finding charts for LMC 1948. The 3-sigma positional error is marked by the circle. LMC 1948 is located on the left hand side (around 8 o’clock) between a bright star and the error circle.
90
M.M. Shara / New Astronomy Reviews 44 (2000) 87 – 91
Fig. 4. Finding charts for LMC 1971b. The 3-sigma positional error is marked by the circle. LMC 1971b is not detected in these images.
Fig. 5. KPNO 0.9m B image of M81. The positions of the novae are indicated and numbered on the image.
M.M. Shara / New Astronomy Reviews 44 (2000) 87 – 91
shows the finding chart for LMC 1948 A.D., a clear detection. Fig. 4 is the chart for LMC 1971b, a clear nondetection. While the CCD plate limits in this survey are 1 to 2 mag fainter than the expected brightnesses of canonical old LMC novae, I don’t place much significance on the modest recovery fraction. The luminosity distribution of the LMC old novae could be compatible with that of the Galactic old novae. This is because crowding is severe in many of the fields, and it is likely that HST and / or 8 meter ground based imagery will be required to recover most of the rest of the quiescent LMC cataclysmics. The recovered objects’ orbital periods are now within reach. This will be an important test of the close binary evolution prediction that this period distribution is similar to that of the Galactic novae.
4. Novae in M81 A five year series of Palomar 5 meter photographic plates of M81 have been analyzed (in
91
collaboration with Allan Sandage and David Zurek). Twenty three novae were detected: they are shown in Fig. 5. The key result of the survey is the remarkable spatial distribution of the erupting cataclysmic binaries. Despite the incompleteness of nova detections in the central arcminute of the galaxy (due to the modest dynamic range of the photographic plates) the prevalence of novae in the spiral arms is evident. This is consistent with theoretical suggestions that massive WDs, descended from relatively massive primaries in spiral arms, should erupt as novae much more frequently than novae with low mass WD primaries.
5. Summary Extragalactic novae are challenging to detect and to characterize. They have much to teach us about cataclysmic binary host populations and stellar evolution. The coming generation of 8 meter telescopes, in conjunction with HST and other space telescopes are the essential tools for carrying out these studies.