What primarily defines the Herbig Ae/Be stars?

Herbig Ae/Be stars are primarily defined by their strong emission lines, which are characteristics of their spectral classification. These stars are a type of young, pre-main-sequence star that exhibits significant activity in their surrounding environment, often resulting in the presence of a circumstellar disk and intense stellar winds.

The strong emission lines observed in their spectra are primarily due to the hydrogen lines, as well as metals, which indicate that these stars are still in the process of forming. The emission lines arise because the high energy and radiation from these young stars ionize the surrounding gas, causing it to emit light at specific wavelengths. This is indicative of the energetic processes occurring as these stars accrete material from their surroundings.

In contrast, strong absorption lines (the first option) are more common in older stars that have settled onto the main sequence and whose spectra are influenced by cooler temperatures and gravity effects rather than ongoing accretion and emission from surrounding material. The option referring to no emission lines is incorrect because Herbig Ae/Be stars are specifically known for their prominent emissions. High temperatures are a feature of these stars, but they are not the defining aspect; the defining feature is the strong emission lines that signify their young, active status in stellar evolution.