Light echoes reveal an unexpectedly cool Eta Carinae during its 19th-century Great Eruption (1112.2210v1)

Abstract: Eta Carinae (Eta Car) is one of the most massive binary stars in the Milky Way. It became the second-brightest star in the sky during its mid-19th century "Great Eruption," but then faded from view (with only naked-eye estimates of brightness). Its eruption is unique among known astronomical transients in that it exceeded the Eddington luminosity limit for 10 years. Because it is only 2.3 kpc away, spatially resolved studies of the nebula have constrained the ejected mass and velocity, indicating that in its 19th century eruption, Eta Car ejected more than 10 M_solar in an event that had 10% of the energy of a typical core-collapse supernova without destroying the star. Here we report the discovery of light echoes of Eta Carinae which appear to be from the 1838-1858 Great Eruption. Spectra of these light echoes show only absorption lines, which are blueshifted by -210 km/s, in good agreement with predicted expansion speeds. The light-echo spectra correlate best with those of G2-G5 supergiant spectra, which have effective temperatures of ~5000 K. In contrast to the class of extragalactic outbursts assumed to be analogs of Eta Car's Great Eruption, the effective temperature of its outburst is significantly cooler than allowed by standard opaque wind models. This indicates that other physical mechanisms like an energetic blast wave may have triggered and influenced the eruption.

Spectroscopic Analysis of η Carinae's Great Eruption Through Light Echoes

The paper outlined in the paper offers an in-depth analysis of η Carinae's 19th-century Great Eruption, leveraging the unique astronomical phenomenon of light echoes. This research provides significant insights into the characteristics of Luminous Blue Variables (LBVs), employing light echoes to infer properties of the historical eruption.

Central to the paper is the discovery of light echoes from η Carinae’s Great Eruption, a mid-19th-century event that saw the star greatly exceed the Eddington luminosity limit without resulting in a supernova. The use of light echoes allows researchers to circumvent the constraints of historical astronomical observations, offering a novel approach to scrutinizing such transient phenomena. Light echoes from this eruption produce spectra showing only absorption lines, blueshifted by -210 km s$^{-1}$, corroborating previously calculated expansion velocities.

A remarkable and somewhat unexpected finding of this paper is the cooler effective temperature of the eruption, registering around $\sim$5000 K which is akin to the spectral characteristics of G2-G5 supergiants. This poses a stark contrast to the standard wind model that predicts an opaque photosphere with temperatures no lower than 7000 K. Thus, the paper points toward mechanisms beyond the traditional radiative models, suggesting the influence of an energetic event such as a blast wave.

From a theoretical standpoint, this information fundamentally challenges the canonical models of LBV outbursts, which have traditionally relied on the notion of an opaque stellar wind driven by a luminosity increase forming an F-type spectrum. The implication of cooler temperatures suggests that alternative processes, possibly involving hydrodynamic explosions, may play a crucial role in understanding such eruptions.

Further insights are derived from a series of Doppler measurements of absorption features, confirming the spectral analysis and offering data on mass ejection velocities. These findings propose a more complex picture of stellar eruptions, characterized by non-standard asymmetries and varying velocities across different latitudinal sections of the erupting star.

For the broader landscape of astrophysical research, these findings emphasize the potential variability and complexity of massive stellar eruptions. The paper, therefore, presents exploration parameters that may direct future research avenues in astrophysics, particularly concerning the predictive modeling of LBV and supernova imposters.

Looking ahead, it would be beneficial to harness the power of light echo analysis to examine further astrophysical transients and to incorporate radiative transfer simulations that can be integrated with these findings to form a comprehensive model describing η Carinae's Great Eruption. Continued observation, especially with modern instrumentation capable of piercing through the nebula’s ambient emission, will allow for more precise constraints on these eruptions and even detection of high-velocity polar ejecta.

In conclusion, this research not only opens new vistas in the paper of variable massive stars but also underscores the transformative potential of applying spectroscopic analysis to historical light echoes. Such an approach stands to redefine our understanding of stellar life cycles and the mechanics underpinning massive stellar explosions.