The evolved system upsilon Sgr (HD 181615, HR 7342, HIP 95176, V = 4.61 mag) includes an A-type low-mass supergiant, which is the brightest member of the type of extremely hydrogen-deficient binaries stars (HdB stars). Jim Kaler gives a nice description of this strange star: http://stars.astro.i...sow/upssgr.html
It's classed often as a binary with a composite spectrum: as a peculiar B2 dwarf plus an A2 supergiant shell star (implying a surrounding disk).
The star is now visible at my observation site, just a few degrees over the rooftops of my neighbour’s houses:
The spectrum, although recorded with my low resolution Star Analyser setup, is highly interesting: The H alpha line shows apparently a P-Cygni profile and some other lines of the Balmer series have apparently strengthened (H gamma is seen in emission ?). A comparison of my spectrum with James Fosters’ spectrum (AAVSO database) from last year illustrates the differences. A spectrum from the BeSS database, that was recorded 2 years ago, also shows a P-Cygni profile (see inset below http://basebe.obspm....hp?v_ids=230676). I could not find a more recent spectrum of ups Sgr to verify the validity of my spectrum (my insensitive, low budget system is prone to artifact formation: Debayering, rotating, stacking are sources of errors):
Are there other recent spectra of ups Sgr available?
The NIR region of the spectrum is "astrophysically" highly interesting:
The presence of Ca II triplet emission in ups Sgr presents an apparent paradox. The lines arise from the 2P to 2D transition (see Grotrian diagram below). The upper level (2P) is the same as for the H and K lines; the lower (2D) level is strongly metastable. The triplet lines are very subordinate transitions to the H and K lines (branching ratio 1: 38). The triplet lines themselves do not appear normal: all have similar intensities rather than the expected 2:18:10 (see inset below showing the Ca II triplet in standard array; the intensities in LS-coupling multiplets were taken from Allen’s Astrophysical Quantities http://minerva.ufsc....Chap/book04.pdf). This implies that the gas giving rise to the emission is optically thick to the triplet. A possible explanation is that ups Sgr showing Ca II triplet emission is indeed actually a semi-detached interacting binary. The source of the dense cool gas is the mass losing contact component and the Ca II triplet emission is probably produced in the very extended atmosphere by radiative de-excitation from the absorption by the Ca II H and K lines in the star’s photosphere.
The forbidden [Ca II] lines are rarely seen. They are also reported in the cool hypergiant IRC +10420 (see spectrum above for comparison from Robin Leadbeater, BAA database). The transition that produces the Ca II triplet leaves the atoms in the upper level for the forbidden lines. These atoms are normally collisionally de-excited back to the ground state that produces the H and K lines. Thus the forbidden [Ca II] lines are produced in a region of much lower density than is normally found even in the atmospheres of the most luminous supergiants (that we are dealing with supergiants is indicated by the O I line in the NIR; see for comparison the spectrum of Sadr (gamma Cyg) in the spectrum above). The [Ca II] emission may also be evidence of a hotter companion who is also responsible for the ionization of hydrogen (H alpha in emission!).
Some interesting literature on these stars:
Edited by mwr, 31 July 2022 - 11:08 PM.