```typescript
/** The temperature in degrees Fahrenheit. */
export function celsiusToFahrenheit(celsius: number): number {
  return convertCelsiusToTemperature(celsius, "F");
}

/** The temperature in kelvin. */
export function celsiusToKelvin(celsius: number): number {
  return convertCelsiusToTemperature(celsius, "K");
}

function convertCelsiusToTemperature(celsius: number, targetUnit: string): number {
  if (typeof celsius !== "number" || !Number.isFinite(celsius)) {
    throw new Error("celsius must be a finite number");
  }
  if (celsius < -273.15) {
    throw new Error("celsius must be at or above absolute zero");
  }

  switch (targetUnit) {
    case "F":
      return celsius * 1.8 + 32;
    case "K":
      return celsius + 273.15;
    default:
      throw new Error(`Unsupported target unit: ${targetUnit}`);
  }
}
```