Brewing another binary in the Teapot: Nandou=Phi Sagittarii
Abstract
The iconic Teapot asterism in Sagittarius is particularly rich in intermediate-mass stars in multiple systems. Here we combine VLTI/GRAVITY, archival spectroscopic observations, past radial velocity measurements and photometry to solve for the orbital and stellar parameters of the binary Nandou=Phi Sagittarii. It has a period P=238.844±0.011 days, a semi-major axis a=1.32±0.10 au and eccentricity e=0.418±0.004. The precision in the dynamical masses MA=3.8±0.9 M⊙ and MB=1.6±0.4 M⊙ is currently limited by the parallax p=12.5±1.0 mas. We find that the original Hipparcos 1997 Intermediate Astrometric Data (IAD) are consistent with the expected photocenter semi-major axis aphot ∼ 5 mas, while the IAD from the Hipparcos 2007 new reduction yield a too small aphot = 1.7±0.3 mas, confirming the suspicion that a too aggressive modeling of the satellite attitude can lead to overfitting that suppresses the orbital motion. Matching the primary's temperature Teff,A = 12000±250 K and radius RA = 5.1± 0.4 R⊙ to stellar evolutionary models implies that it is a MA ≃ 4 M⊙ star near the end of the main-sequence. Nandou will eventually most likely become a symbiotic star.
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BibTeX
@misc{waisberg2026brewing,
title = {{Brewing another binary in the Teapot: Nandou=Phi Sagittarii}},
author = {Idel Waisberg and Boaz Katz},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2611.05455},
eprint = {2611.05455},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2611.05455},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Brewing another binary in the Teapot: Nandou=Phi Sagittarii
AU - Idel Waisberg
AU - Boaz Katz
PY - 2026
DA - 2026/11/03
PB - arXiv
DO - 10.48550/arxiv.2611.05455
UR - https://arxiv.org/abs/2611.05455
AB - The iconic Teapot asterism in Sagittarius is particularly rich in intermediate-mass stars in multiple systems. Here we combine VLTI/GRAVITY, archival spectroscopic observations, past radial velocity measurements and photometry to solve for the orbital and stellar parameters of the binary Nandou=Phi Sagittarii. It has a period P=238.844±0.011 days, a semi-major axis a=1.32±0.10 au and eccentricity e=0.418±0.004. The precision in the dynamical masses M_A=3.8±0.9 M_⊙ and M_B=1.6±0.4 M_⊙ is currently limited by the parallax p=12.5±1.0 mas. We find that the original Hipparcos 1997 Intermediate Astrometric Data (IAD) are consistent with the expected photocenter semi-major axis a_phot ∼ 5 mas, while the IAD from the Hipparcos 2007 new reduction yield a too small a_phot = 1.7±0.3 mas, confirming the suspicion that a too aggressive modeling of the satellite attitude can lead to overfitting that suppresses the orbital motion. Matching the primary's temperature T_eff,A = 12000±250 K and radius R_A = 5.1± 0.4 R_⊙ to stellar evolutionary models implies that it is a M_A ≃ 4 M_⊙ star near the end of the main-sequence. Nandou will eventually most likely become a symbiotic star.
ER -