maravelias.info

This is a paper that I finally managed to complete. Starting back in 2016 we looked into the light curves for ρ Cas to identify potential correlations with its latest outburst in 2013, but not all data made it through the final paper (Kraus et al. 2019). Given this first analysis and the fact that visual observations cover almost a century of star’s behavior, we continued the study and we looked into the four distinct outbursts. The result is even more interesting as there is a clear trend of shorter and more frequent outbursts, as if ρ Cas is bouncing against the Yellow Void.

Bouncing against the Yellow Void — exploring the outbursts of ρ Cas from visual observations

Grigoris Maravelias and Michaela Kraus

Massive stars are rare but of paramount importance for their immediate environment and their host galaxies. They lose mass from their birth through strong stellar winds up to their spectacular end of their lives as supernovae. The mass loss changes as they evolve and in some phases it becomes episodic or displays outburst activity. One such phase is the Yellow Hypergiants, in which they experience outbursts due to their pulsations and atmosphere instabilities. This is depicted in photometry as a decrease in their apparent magnitude. The object ρ Cassiopeia (Cas) is a bright and well known variable star that has experienced four major outbursts over the last century, with the most recent one detected in 2013. We derived the light curves from both visual and digital observations and we show that with some processing and a small correction (∼0.2 mag) for the visual the two curves match. This highlights the importance of visual observations both because of the accuracy we can obtain and because they fully cover the historic activity (only the last two of the four outbursts are well covered by digital observations) with a homogeneous approach. By fitting the outburst profiles from visual observations we derive the duration of each outburst. We notice a decreasing trend in the duration, as well as shorter intervals between the outbursts. This activity indicates that ρ Cas may be preparing to pass to the next evolutionary phase.

arXiv: 2112.13158

Evolved Massive Stars at Low-metallicity II. Red Supergiant Stars in the Small Magellanic Cloud

Ming Yang, Alceste Z. Bonanos, Bi-Wei Jiang, Jian Gao, Panagiotis Gavras, Grigoris Maravelias, Shu Wang, Xiao-Dian Chen, Frank Tramper, Yi Ren, Zoi T. Spetsieri, Meng-Yao Xue

We present the most comprehensive RSG sample for the SMC up to now, including 1,239 RSG candidates. The initial sample is derived based on a source catalog for the SMC with conservative ranking. Additional spectroscopic RSGs are retrieved from the literature, as well as RSG candidates selected from the inspection of CMDs. We estimate that there are in total ∼ 1,800 or more RSGs in the SMC. We purify the sample by studying the infrared CMDs and the variability of the objects, though there is still an ambiguity between AGBs and RSGs. There are much less RSGs candidates (∼4%) showing PAH emission features compared to the Milky Way and LMC (∼15%). The MIR variability of RSG sample increases with luminosity. We separate the RSG sample into two subsamples (“risky” and “safe”) and identify one M5e AGB star in the “risky” subsample. Most of the targets with large variability are also the bright ones with large MLR. Some targets show excessive dust emission, which may be related to previous episodic mass loss events. We also roughly estimate the total gas and dust budget produced by entire RSG population as ∼1.9(+2.4/−1.1)×10⁻⁶ M⊙/yr in the most conservative case. Based on the MIST models, we derive a linear relation between Teff and observed J−KS color with reddening correction for the RSG sample. By using a constant bolometric correction and this relation, the Geneva evolutionary model is compared with our RSG sample, showing a good agreement and a lower initial mass limit of ∼7 M⊙ for the RSG population. Finally, we compare the RSG sample in the SMC and the LMC. Despite the incompleteness of LMC sample in the faint end, the result indicates that the LMC sample always shows redder color (except for the IRAC1−IRAC2 and WISE1−WISE2 colors due to CO absorption) and larger variability than the SMC sample.

arXiv.org: 2005.10108

Just some useful numbers and units to remember and to have an easy access to!
[last updated: 29 Dec 2011]

>> Distance

1 AU = 150 x 10⁶ km

1 pc = 3.09 × 10¹³ km = 206260 AU = 3.26 ly

1 ly = 9.4 x 10¹² km = 64 x 10³ = 0.31 pc

>> Sun

L_☉ = 3.86 x 10²⁶ W = 3.86 x 10³³ erg / s

M_V,☉ = M_bol,☉ = 4.8

M_☉ = 2 x 10³⁰ kg

R_☉ = 6.96 x 10⁵ km

V_☉,relative = 30 km/s

T_☉ = 5780 K

R_Sun-GC = 8 kpc

t_{galactic orbit} = 250 x 10⁶ yr

V_{galactic rotation} = 200 km/s

solar wind’s energy ~ 10^-7 L_☉ (on average)

solar wind’s density (in the vicinity of Earth) ~ 5 particles / cm³

solar wind’s velocity (in the vicinity of Earth) ~ 400 km / s

mass loss ~ 10^-14 M_☉ / yr

>> Earth

M_Earth = 5.974 x 10²⁴ kg

R_Earth = 6371 km

ρ_Earth = 5.5 g/cm³

>> Milky Way / Galaxy

bulge_radius ~ 5 kpc

disk_radius ~ 15 kpc

halo_radius > 75 kpc (perhaps 100 kpc)

dust ± 300 pc from disk level

stars ± 1000 pc from disk level

M_MW = -20.5 ( 25 mag more than M_Sun, 10¹⁰ times that of Sun)

>> Ionization potential

for H = 13.6 eV

for He = 24.6 eV (one electron) + 54.4 eV (both electrons)

>> Hydrogen atom

R = 1 Å

>> Extinction

Sun’s neighbourhood: ~ 1 mag/kpc

towards GC: ~ 21 mag/kpc – 1 optical photon per 10¹² reaches us, while 1 out of 10 in IR

>> magnitude difference: 5 mags = 100 times in flux [m₁-m₂=-2.5 log₁₀(f₁/f₂)]