angular2 style guide – property with dollar sign?

Parent and children communicate via a service example from the official guide on Angular.io makes use of dollar signs in Observable stream names.

Notice missionAnnounced$ and missionConfirmed$ in the following example:

import { Injectable } from '@angular/core';
import { Subject } from 'rxjs';

@Injectable()
export class MissionService {

  // Observable string sources
  private missionAnnouncedSource = new Subject<string>();
  private missionConfirmedSource = new Subject<string>();

  // Observable string streams
  missionAnnounced$ = this.missionAnnouncedSource.asObservable();
  missionConfirmed$ = this.missionConfirmedSource.asObservable();

  // Service message commands
  announceMission(mission: string) {
    this.missionAnnouncedSource.next(mission);
  }

  confirmMission(astronaut: string) {
    this.missionConfirmedSource.next(astronaut);
  }
}

Can anyone explain:

• why $ is used? What’s the reason behind this notation? Do I always need to use this for public properties?
• public properties are used but not methods (e.g. missionAnnouncements(), missionConfirmations()) – again, is this a convention for Angular2 apps?