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Details about the Discovery of a Possible Luminous Blue Variable in NGC 4414

Lisa M. Frattare and David R. Zurek

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(a) Introduction (Abstract)

We report the serendipitous discovery of a variable object, based on WFPC2 images of NGC 4414 retrieved from the HST archive. Photometric light curves show an increase by more than 1.5 magnitudes over a time scale of a few weeks.

The initial observations were taken for the HST Key Project (Turner et al. 1998) in April - June, 1995. In observations taken one year later, the object was not detected above the background (i.e. the spiral arms; V = ~23 mag). The peak brightness of this object, using a recently determined distance modulus to NGC 4414 (Turner, et al. 1998) of 31.41 mag, is of order MV = -11.5 mag, MB = -12.1 mag.

Follow-up images of NGC 4414 were taken for the Hubble Heritage Project in April, 1999. At this time another peak brightness was measured: MV = -10.4 mag, MB = -10.0 mag, MI = -10.9 mag.

Given the limitations of this dataset it is difficult at present to determine conclusively the nature of this object. However, due to the shape of the light curve and peak brightness values, both a supernova and nova event has been ruled out. An eruption of a Luminous Blue Variable (LBV) seems likely.

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(d) The Case for an LBV

A review of LBV's has shown a class of objects with three specific signatures evident in their light curves: micro-variations, moderate eruptions and giant eruptions. Micro-variations of less than a few tenths of a magnitude have been detected on the time scale of weeks to months. Moderate eruptions show larger luminosity variations on the order of 1 - 2 magnitudes and occur on the time scale of years to decades. Giant eruptions have increases of more than three magnitudes and may last for decades.

With the current observations being presented of the variable object in NGC 4414, we see distinct similarities to several of these signatures:

  • Variations of order 0.5 mags are present in the first two months of observations.

  • A large amplitude increase of 1.5 mags occurs in 15 days.

  • It fades by at least 3 magnitudes from the peak brightness in approximately 10 months (The measured magnitude is an upper limit due to the presence of the spiral arm).

  • Another observation 3 years later shows a visible object slightly fainter than the previous peak brightness.

Although several inferences on the periods and fluctuations in the light curve may be an observational bias due to the dates of observations, there is convincing evidence that the micro- variations are real and thus, representative of the nature of LBV's. As well, the strong fluctuation of nearly three magnitudes on the time scale of several years may be indicative of a moderate eruption.

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(e) Acknowledgments

We would like to thank the following people for ground- and space-based images: Howard Bond, Carol Christian, Jayanne English, Forrest Hamilton, Anne Kinney, Zolt Levay, Keith Noll (Hubble Heritage Team); Wendy Freedman, Jeremy Mould, Robert Kennicutt, Barry Madore (HST Key Project); Jay Gallagher, Linda Smith (4.2 m WHT); And the following folks for consultation: Nino Panagia, Francesca Boffi, Mike Shara, Mario Livio, Antonella Nota (STScI).

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