V615 Vul shows rare hybrid nova signature after rapid two-day rise

Italian astronomers have performed extensive spectroscopic monitoring of a recently discovered nova known as Vulpeculae 2024, also known as V615 Vul. Results of the new observations, presented in a paper published in the Astronomy & Astrophysics journal, shed more light on the nature of Vulpeculae 2024, suggesting that it represents a rare class of hybrid novae.

Novae: Origins and classification

A nova is a star experiencing a sudden increase in brightness and slowly returning to its original state, a process that could last many months. Such an outburst, which releases an immense amount of energy, is the result of the accretion process in a close binary system containing a white dwarf and its companion. Studying novae is crucial in advancing our knowledge about fundamental astrophysical processes, including stellar evolution.

Classical novae are powered by a sudden thermonuclear runaway of hydrogen-rich material that is accreted onto the surface of a white dwarf from a low-mass companion in a close binary system. They are divided into two main spectroscopic classes: those characterized by prominent iron (Fe II) emission lines and those dominated by helium/nitrogen (He/N) emission lines. However, observations show that a small fraction of classical novae exhibit hybrid behavior, evolving from the Fe II to the He/N class.

Knowing nova Vulpeculae 2024 better

Nova Vulpeculae 2024 was discovered on July 29, 2024, just a few hours before passing through photometric maximum. Based on its optical spectrum, it was initially classified by astronomers as a reddened classical nova.

A team of astronomers led by Paolo Valisa of the Asiago Astrophysical Observatory in Italy, has initiated a follow-up monitoring of Nova Vulpeculae 2024, which covered the period starting from its detection to the nova’s nebular phase. For this purpose, they obtained spectra of this nova using the Varese Schiaparelli Observatory. Their study was complemented by data from the American Association of Variable Star Observers (AAVSO) database and from the Zwicky Transient Facility (ZTF).

Fast and hybrid

The observations found that Vulpeculae 2024 is a fast nova as its rise time to maximum brightness was less than two days, while the time required to decline by two and three magnitudes from maximum brightness was measured to be five and 10.7 days, respectively.

In general, the light curve of this nova appears to resemble the oscillating novae—with an initially smooth decline and quasi-periodic oscillations of approximately 1.0 magnitude amplitude.

The distance to Vulpeculae 2024 was calculated to be about 16,000 light years, which places it approximately 390 light years below the galactic plane and within the Orion-Cygnus spiral arm. The astronomers suppose that a star designated GAIA DR3 1825912166611947136 may be a progenitor of this nova.

Most importantly, the observations found that Vulpeculae 2024’s spectrum around maximum brightness was typical of the FeII-class, with very broad emission lines and high-velocity P-Cygni absorption reaching terminal velocities of about −3,500 km/s.

However, when the nova declined three magnitudes from maximum brightness, it began experiencing photometric oscillations primarily driven by changes in the continuum and the emergence of hard X-ray emission, while emission lines typical of the He/N-class began to develop in parallel to those of the FeII-class. Such behavior, according to the authors of the study, confirms that Vulpeculae 2024 is a hybrid nova.

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