The Historical Record of η Carinae I. The Visual Light Curve, 1595-2000

Eta Carinae is one of the most massive luminous blue variable stars (LBVs) in the Milky Way. Here a new historic light curve is presented which supersedes earlier works, and is based almost completely on a critically-assessed compilation of primary sources. Only observations made relative to unambiguously identified comparison stars are used and, unlike earlier efforts, the data are reduced homogenously to the V scale. The light curve is extended through to the present using a database of photoelectric and CCD measures compiled elsewhere, and totals about 1500 points, including about 500 before 1950. A primary motivation for this work is to provide a definitive light curve for eta Car over the maximum time-line possible, to be used as a resource for further investigation.The observation by Keyser leads to an inferred magnitude of mV = 3.5 ± 0.5 in 1595-96, while new magnitude estimates for 1677, 1687, and 1752 are mV = 3.3 ± 0.3, mV = 3.4 ±0.4, and mV = 2.3 ± 0.3 respectively. Contrary to accepted wisdom, there is only weak observational evidence to show that eta Car was varying markedly prior to the Great Eruption beginning in the 1830s, though this is not ruled out. A slow rise from mV approx. 3.5 at the start of the 17th century to mV approx. 2.3 around 1750, with a further slow brightening to around 1830, is also a tenable hypothesis. There is, however, no evidence for a long-duration eruption having occurred between 1500 and1800. The well-known Great Eruption commenced in 1837 and lasted until at least 1856. The brightness declined rapidly between 1858 and 1868, before a small brightening or plateau occurred in 1869-71; there is some evidence for a change in the spectrum in Feb 1871. The Lesser Eruption occurred in 1887-95, the onset of which was within two months of a predicted spectroscopic event, suggesting a causal link. A preliminary look at the photographic magnitudes suggests a marked change in colour index during the Lesser Eruption. Another increase in the integrated brightness is generally believed to have occurred circa 1940, but the exact time and quantum still needs to be ascertained. There is no compelling evidence to suggest there was an eruptive event at this time, but there is good, albeit circumstantial, evidence to indicate that this brightening is simply due to the lobes of the Homunculus becoming visible for the first time. Such a brightening would increase the integrated magnitude by a measurable amount.