HD 188101
A rotating Bp star with chemical spots and weak magnetism.
HD 188101 is a B-type star in the constellation Cygnus, roughly 430 parsecs from Earth. It is classified as a chemically peculiar He-weak SiTiSr star, meaning its atmosphere shows a moderate deficiency of helium and strong overabundances of silicon, titanium, and strontium. Its surface gravity (log g) of 3.70 cgs indicates the star has left or is leaving the main sequence. The star rotates with a period of 3.98726 days.
Analysis of the star's atmospheric composition, accounting for non-local thermodynamic equilibrium effects, reveals significant deviations from solar abundances. Helium is moderately depleted, with a log(He/H) value below the solar level. Silicon is notably overabundant, with log(Si/H) ≈ −3.74. Titanium and strontium lines are strongly enhanced, which is the defining characteristic of the SiTiSr group. Carbon and oxygen abundances are close to solar, while magnesium and iron-peak elements show uneven distributions and local excesses.
The star’s surface is marked by large-scale chemical spots. Spectral variability, driven by rotation, shows a stable anticorrelation: variations in helium (He I) and magnesium (Mg II) lines are out of phase with those of silicon (Si II, Si III), titanium (Ti II), and iron (Fe II), indicating spatially separated helium-magnesium and silicon-titanium regions. Vertical stratification is also present, with different ions of the same element showing concentration gradients through the atmosphere.
Like most chemically peculiar Bp stars, HD 188101 hosts a global magnetic field that suppresses convection and promotes radiative diffusion. However, its field is relatively weak for this class, estimated at less than 1 kG. The star belongs to the Alpha2 Canum Venaticorum (ACV) variable class. Data from Kepler/K2 and TESS missions confirm strictly periodic brightness variations with an amplitude of about 0.02 magnitudes, matching the rotation period and caused by the passage of chemical inhomogeneities across the star’s disk.
- Spectral type
- B
- Distance
- 430 pc
- Surface gravity (log g)
- 3.70 cgs
- Rotation period
- 3.98726 days
- Helium abundance (log(he/h))
- lower than solar
- Silicon abundance (log(si/h))
- −3.74
- Magnetic field strength
- less than 1 kG
Lore & Background
HD 188101 is a chemically peculiar star of spectral type B, located in Cygnus at a distance of 430 pc. Its surface gravity of log g = 3.70 cgs indicates it has left or is leaving the initial main sequence. The star exhibits a moderate helium deficiency and a significant silicon overabundance (log(Si/H) ≈ −3.74), while titanium and strontium lines are strongly enhanced, defining its classification as a SiTiSr object. Carbon and oxygen abundances are near solar, while magnesium and iron peak elements show uneven distributions with local excesses.
The star possesses a global magnetic field, though it is relatively weak for chemically peculiar stars, estimated at less than 1 kG. This field suppresses convection and promotes radiative diffusion, leading to an inhomogeneous distribution of chemical elements across the photosphere. Spectral variability arises from rotation, with helium and magnesium lines varying out of phase with silicon, titanium, and iron lines, indicating spatially separated chemical regions. Vertical stratification of elements is also observed.
Data from the Kepler/K2 and TESS space missions confirm strictly periodic brightness variability with an amplitude of about 0.02 magnitudes, exactly matching the rotation period of 3.98726 days. This photometric variation is caused by the geometric passage of chemical inhomogeneities across the stellar disk. The star is classified as an Alpha2 Canum Venaticorum (ACV) variable.
Reader's Guide
HD 188101 is significant as a member of the He-weak SiTiSr subclass of chemically peculiar Bp stars, illustrating the interplay between weak magnetic fields, radiative diffusion, and rotational modulation. Its relatively weak magnetic field (less than 1 kG) distinguishes it from many other CP stars, yet it still drives the chemical inhomogeneities that produce its variability. The anticorrelation between helium-magnesium regions and silicon-titanium regions provides a clear example of how magnetic fields can segregate elements across a stellar surface. The star's photometric period, exactly equal to its rotation period, demonstrates that brightness variations in such objects are purely geometric, arising from the rotation of spotted surfaces. The detection of vertical stratification of elements adds further complexity to the understanding of chemical peculiarity. As an ACV variable, HD 188101 contributes to the broader study of magnetic chemically peculiar stars and their evolution off the main sequence, as indicated by its surface gravity.
Did You Know?
- HD 188101 has a rotation period of 3.98726 days.
- Its magnetic field is less than 1 kG, weak for a chemically peculiar star.
- Helium and magnesium lines vary out of phase with silicon, titanium, and iron lines.
- The star's brightness varies by about 0.02 magnitudes due to rotating chemical spots.
Frequently Asked Questions
What is HD 188101?
HD 188101 is a B-type chemically peculiar star located in the constellation Cygnus, roughly 430 parsecs from Earth. It belongs to the He-weak SiTiSr subclass, meaning its outer layers show a noticeable helium shortfall alongside pronounced excesses of silicon, titanium, and strontium.
What makes HD 188101's atmosphere unusual compared to the Sun?
The star's helium-to-hydrogen ratio sits below the solar value, while silicon (log Si/H ≈ −3.74) and other heavy elements are significantly overabundant. Non-local thermodynamic equilibrium modeling of its spectrum reveals these deviations are far larger than what a simple solar-composition model would predict.
Is HD 188101 still a main-sequence star?
Its measured surface gravity of log g ≈ 3.70 cgs suggests it has already departed, or is in the process of departing, the main sequence. That intermediate gravity places it in a transitional evolutionary phase rather than a stable hydrogen-burning stage.
How does HD 188101 produce its variability?
As a rotating Bp star, it carries persistent chemical spots on its surface and hosts a weak magnetic field that helps segregate elements. The roughly 3.99-day rotation period means those inhomogeneities sweep into and out of view, producing the observed photometric and spectroscopic changes.
Why do variable-star fans care about HD 188101?
It serves as a relatively nearby, well-characterized example of how weak magnetism and rotation can sculpt atmospheric chemistry in B-type stars. Its clean, periodic signal and detailed abundance measurements make it a useful benchmark for testing non-LTE and rotational-mixing models.
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