HD 137509
A helium-weak Bp star with a 29 kG magnetic field.
HD 137509, also designated NN Apodis, lies in the southern constellation Apus, less than a degree from the border with Triangulum Australe. This star varies in apparent visual magnitude between 6.86 and 6.93 over a period of 4.4916 days. Based on parallax measurements, it is roughly 647 light-years from the Sun and is moving away at a radial velocity of +0.50 km/s.
Identified in 1973 as a peculiar A star of the silicon type, its spectrum was later reclassified in 1975, with hydrogen lines stronger and helium lines weaker than typical for a class near B9. Its luminosity lies above the main sequence, common for peculiar A stars. The stellar atmosphere is helium-poor but rich in metallic lines, though it lacks manganese and mercury, ruling out a Hg–Mn Ap star. It is now classified as B9p (SiCrFe) or B8 He wk SiCrFe—a late-type, helium-weak Bp star with excess silicon, chromium, and iron.
Photometric variability was detected in 1989, and spectral variability was noted a decade earlier. The star possesses one of the strongest magnetic fields known among chemically peculiar stars, around 29 kG, with a pronounced quadrupolar component. Both the brightness and spectral changes allow precise measurement of its rotation period. It is categorized as an Alpha2 Canum Venaticorum variable. HD 137509 is about 124 million years old, with 3.4 times the Sun’s mass and 2.8 times its radius, radiating roughly 123 times the Sun’s luminosity from a photosphere at an effective temperature of 13,100 K.
- Variable star designation
- NN Apodis (NN Aps)
- Apparent visual magnitude range
- 6.86 to 6.93
- Period
- 4.4916 days
- Distance
- approximately 647 light years
- Radial velocity
- +0.50 km/s
- Spectral type
- B9p (SiCrFe) or B8 He wk SiCrFe
- Magnetic field strength
- around 29 kG
- Mass
- 3.4 times the Sun
- Radius
- 2.8 times the Sun
- Luminosity
- ~123 times the Sun
Lore & Background
In 1973, W. P. Bidelman and D. J. MacConnell identified HD 137509 as a peculiar A star of the silicon type. During a 1975 reclassification by A. P. Cowley and N. Houk, the strength of hydrogen lines and weakness of helium suggested a class near B9, with a luminosity above the main sequence common for peculiar A stars. The stellar atmosphere is deficient in helium but shows a rich variety of metallic lines, though no lines of manganese or mercury, ruling out a Hg–Mn Ap star. It is now classified as B9p (SiCrFe) or B8 He wk SiCrFe, a late-type, helium-weak Bp star with overabundances of silicon, chromium, and iron.
The star was found to be photometrically variable by L. O. Lodén and A. Sundman in 1989, and a variable spectrum was noted by H. Pedersen in 1979. It has one of the strongest magnetic fields recorded for a chemically peculiar star, around 29 kG, with a strong quadrupolar component. Both variances allow its rotation period to be precisely measured. It is classified as an Alpha2 Canum Venaticorum variable. The star is about 124 million years old, with 3.4 times the mass of the Sun and 2.8 times the Sun's radius, radiating ~123 times the Sun's luminosity at an effective temperature of 13,100 K.
Reader's Guide
HD 137509 is significant as a benchmark for studying chemically peculiar stars with extreme magnetic fields. Its classification as an Alpha2 Canum Venaticorum variable, combined with a measured magnetic field of around 29 kG—one of the strongest recorded for such stars—makes it a key object for understanding the relationship between magnetic fields, chemical peculiarities, and rotational variability. The star's helium weakness and overabundances of silicon, chromium, and iron, alongside the absence of manganese and mercury, help refine the taxonomy of Bp stars. Its precisely measured rotation period of 4.4916 days, derived from both photometric and spectral variations, provides a reliable baseline for modeling magnetic field geometry and surface abundance patches. The star's distance of about 647 light years and radial velocity of +0.50 km/s indicate it is drifting away from the Sun. Its age of roughly 124 million years and mass of 3.4 solar masses place it in a stage of evolution above the main sequence, typical for such peculiar stars. The legacy of HD 137509 lies in its role as a well-characterized example of a magnetic, helium-weak Bp star, contributing to the broader understanding of stellar magnetism and chemical diffusion processes.
Did You Know?
- It is classified as an Alpha2 Canum Venaticorum variable with a period of 4.4916 days.
- The star is deficient in helium but shows overabundances of silicon, chromium, and iron.
Frequently Asked Questions
What is HD 137509?
HD 137509 is a variable star in the southern constellation Apus, carrying the variable-star name NN Apodis (NN Aps). It sits less than a degree from the border with Triangulum Australe and is roughly 647 light-years from the Sun.
What type of variable star is HD 137509?
It is a helium-weak Bp star with a magnetic field of about 29 kilogauss. Its spectrum shows enhanced silicon, chromium, and iron lines alongside weaker-than-normal helium, giving it the designations B9p (SiCrFe) or B8 He wk SiCrFe.
How does HD 137509 vary in brightness?
The star's apparent visual magnitude oscillates between 6.86 and 6.93 over a period of approximately 4.4916 days. This modest amplitude is consistent with the slow rotational modulation expected of a magnetic chemically peculiar star.
Where is HD 137509 in the sky and how far away is it?
It lies in Apus, the bird constellation of the southern sky, just under one degree from Triangulum Australe. Parallax measurements place it at about 647 light-years, and it is receding from the Sun at a radial velocity of +0.50 km/s.
How was HD 137509's spectral type determined?
The star was first identified in 1973 as a peculiar A-type silicon star, but a 1975 reclassification showed hydrogen lines stronger and helium lines weaker than expected for that temperature class. This led to its current placement as a late-B helium-weak magnetic star rather than a true A-type object.
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