HD 344787: a true Polaris analogue?^⋆

¹ INAF-Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Naples, Italy
e-mail: vincenzo.ripepi@inaf.it
² INAF-Osservatorio Astrofisico di Catania, Via S.Sofia 78, 95123 Catania, Italy
³ Konkoly Observatory, ELKH Research Centre for Astronomy and Earth Sciences, Konkoly Thege 15-17, 1121 Budapest, Hungary
⁴ MTA CSFK Lendület Near-Field Cosmology Research Group, Budapest, Hungary
⁵ ELTE Eötvös Loránd University, Institute of Physics, 1117, Pázmány Péter sétány 1/A, Budapest, Hungary
⁶ INAF-Osservatorio di Astrofisica e Scienza dello Spazio, Via Gobetti 93/3, 40129 Bologna, Italy
⁷ Dipartimento di Fisica “E. Pancini”, Universitá di Napoli “Federico II”, Via Cinthia, 80126 Napoli, Italy
⁸ Istituto Nazionale di Fisica Nucleare (INFN)-Sez. di Napoli, Via Cinthia, 80126 Napoli, Italy
⁹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei Munchen, Germany
¹⁰ Cluster of Excellence Universe, Technical University of Munich, Boltzmannstr 2, 85748 Garching, Germany
¹¹ Ludwig-Maximilians-Universitat München, Physics Department, Geschwister-Scholl-Platz 1, 80539 München, Germany
¹² INAF-Osservatorio Astronomico di Roma, via Frascati 33, Roma, Italy

Received: 13 December 2020
Accepted: 20 January 2021

Abstract

Context. Classical Cepheids (DCEPs) are the most important primary indicators for the extragalactic distance scale, but they are also important objects in their own right, allowing us to place constraints on the physics of intermediate-mass stars and the pulsation theories.

Aims. We have investigated the peculiar DCEP HD 344787, which is known to exhibit the fastest positive period change of DCEPs, along with a quenching amplitude of the light variation.

Methods. We used high-resolution spectra obtained with HARPS-N at the TNG for HD 344787 and the more famous Polaris DCEP to infer their detailed chemical abundances. Results from the analysis of new time-series photometry of HD 344787 obtained by the TESS satellite are also reported.

Results. The double-mode nature of the HD344787 pulsation is confirmed by an analysis of the TESS light curve, although with rather tiny amplitudes of a few dozen millimag. This is indication that HD344787 is on the verge of quenching the pulsation. Analysis of the spectra collected with HARPS-N at the TNG reveals an almost solar abundance and no depletion of carbon and oxygen. This means that the star appears to have not gone through first dredge-up. Similar results are obtained for Polaris.

Conclusions. Polaris and HD344787 are both confirmed to be most likely at their first crossing of the instability strip. The two stars are likely at the opposite borders of the instability strip for first-overtone DCEPs with metal abundance Z = 0.008. A comparison with other DCEPs that are also thought to be at their first crossing allows us to speculate that the differences we see in the Hertzsprung-Russell diagram might be due to differences in the properties of the DCEP progenitors during the main-sequence phase.